Copycat Costco Sheet Cake Recipe: Why Shortcuts Are the Point

By the end of this guide, you'll have a two-layer vanilla sheet cake filled with vanilla cheesecake mousse and covered in bakery-style frosting that replicates the Costco original. No membership required. Delish puts the total time at 1 hour 25 minutes, the yield at 15 to 18 servings, and the ingredient cost under $25 based on their own test kitchen run. Two 13x9-inch pans, one party cake.

The reason most copycat Costco sheet cake attempts fall flat isn't that they cut corners. It's that they cut the wrong ones. Delish developed this recipe by ordering an actual Costco cake and tasting each component separately and together. What they found is that Costco's cake doesn't taste the way it does despite convenience products. It tastes that way because of them. A doctored boxed mix, instant pudding added straight to the bowl, vegetable shortening beaten into the frosting. Those are the method. This guide is built around that finding.

A note on the frosting before you start: there are three viable paths, and the choice changes the result. For the closest match to the warehouse original, the all-shortening frosting is the recommendation here. For something richer, a half-butter, half-shortening hybrid works. For a fully butter-forward result, The Slow Roasted Italian offers a from-scratch version using close to two pounds of butter, no shortening but that's a different cake. This guide follows the all-shortening path, with the other options noted where they diverge.

What you need before you start: Stand mixer or hand mixer with a large bowl. Two 13x9-inch metal baking pans. Offset spatula. Piping bag and star tip (optional, but required for the shell border). Wire rack. Parchment paper.

How to make a Costco sheet cake that actually tastes right: breaking down the three components

Before the steps, understand why each component works the way it does. Every ingredient choice here has a specific job.

The cake: Boxed white cake mix, doctored. Not because it's faster, though it is, but because the texture is right. Delish found that replacing the water in the standard box directions with whole milk and sour cream produces a slightly denser, more structured crumb. Egg whites only (no yolks) keep the interior white. Salt and clear vanilla sharpen the flavor. The Slow Roasted Italian and 12 Tomatoes both use from-scratch cake bases with cake flour and butter. Valid approaches, but they take more technique and time.

The mousse: Cream cheese, instant vanilla pudding, milk, and heavy cream. Delish concluded through direct component tasting that Costco's filling is essentially a cream-cheese-and-pudding mixture lightened with whipped cream not a true culinary mousse. The critical detail: use instant pudding only, pour it directly into the bowl with 1 cup of milk, and ignore the package directions entirely. More milk makes it too loose to hold between layers. 12 Tomatoes uses a similar cream cheese, whipped cream, milk, and pudding base for the filling, which puts the basic structure on solid footing across sources.

The frosting: This is where you choose. Delish says its testers found that the most authentically Costco-tasting frosting is pure vegetable shortening (Crisco) beaten with water, vanilla, and confectioners' sugar. No butter. The result is that clean, white, sweet bakery frosting. It's also cheaper than butter and requires no softening time. The Slow Roasted Italian goes all-butter, producing something richer and more complex but further from the warehouse original. 12 Tomatoes splits the difference with equal parts butter and shortening.

Decision guide:

• Closest to Costco: All-shortening (Crisco + water + confectioners' sugar + vanilla + salt)
• Best compromise: Half butter, half shortening more flavor depth, some bakery character
• Most homemade: All butter choose this if replicating Costco is less important than richness

Ingredient quantities for the butter-based versions are at the linked sources above.

The recipe: cake layers, mousse, frosting, and assembly

Ingredients

Cake layers:

• 2 boxes (15.25 oz each) white cake mix
• 8 large egg whites
• 1½ cups sour cream
• 1 cup whole milk
• ⅔ cup vegetable oil
• 2 tsp pure vanilla extract (clear preferred)
• ½ tsp kosher salt

Vanilla cheesecake mousse:

• 1 block (8 oz) cream cheese, room temperature
• ½ cup confectioners' sugar
• ½ tsp kosher salt
• 1 cup whole milk
• 1 package (3.4 oz) instant vanilla pudding mix
• 1 cup heavy cream

All-shortening frosting:

• 2 cups vegetable shortening (Crisco)
• ½ cup water
• 1 tbsp clear vanilla extract
• ¾ tsp kosher salt
• 8 cups confectioners' sugar

Sprinkles for decorating (optional)

Step 1. Prep the pans and preheat. Set a rack in the center of the oven and preheat to 350°F. Grease two 13x9-inch metal baking pans with cooking spray. Line each with parchment paper, leaving overhang on the long sides you'll use this to lift the layers out cleanly later.

Step 2. Mix the cake batter. Whisk the cake mix and salt together in a large bowl. Add the egg whites, sour cream, milk, oil, and vanilla. Beat on medium speed until smooth and lump-free, about 2 minutes. The batter will be noticeably thicker than standard boxed-mix batter Delish notes it won't level itself, so use a spatula to spread it evenly across each pan.

Step 3. Bake. Bake 20 to 25 minutes, until the tops are lightly golden, the edges pull away from the pan sides, and the center springs back when pressed. Start checking at 20 minutes. A toothpick inserted in the center should come out clean.

Step 4. Cool the layers. Let the cakes rest in the pans for at least 10 minutes, then lift out using the parchment and transfer to a wire rack. The layers must be completely cool before the mousse goes on warm cake will liquefy it. Delish lists total time at 1 hour 25 minutes, which assumes efficient cooling; refrigerating the layers after the pan rest speeds things up considerably.

Step 5. Level the cakes. Once cool, use a serrated knife to trim any domed tops flat. Even layers are what give the finished cake its clean, stacked profile.

Step 6. Make the mousse. Beat the cream cheese, confectioners' sugar, and salt together until completely smooth, 1 to 2 minutes. Scrape the bowl. Add the milk and the entire pudding packet directly do not follow the box instructions. Beat until combined, about 1 minute. Add the heavy cream and beat until the filling is thick and light, 1 to 2 more minutes. It should hold its shape when spread. Per Delish, the mousse is best used immediately after mixing make it the day of assembly, not ahead.

Step 7. Make the frosting. Beat the shortening, water, vanilla, and salt on medium-high until smooth and creamy. Add the confectioners' sugar one cup at a time, beating on low after each addition. Once all 8 cups are incorporated, increase to medium-high and beat until smooth and fluffy, about 1 more minute. Transfer one-quarter of the frosting to a piping bag fitted with a star tip and set aside.

Step 8. Assemble. Place one trimmed cake layer on your serving platter, trimmed side up. Spread the mousse in an even layer across the surface, leaving a half-inch border at all edges it will press outward slightly when the second layer goes on. Per The Slow Roasted Italian, refrigerate the mousse-topped layer for 10 to 15 minutes before placing the second cake on top. This brief chill firms the mousse enough that the layers won't slide when you apply the outer frosting. Place the second cake layer trimmed side down.

Step 9. Frost and decorate. Use the offset spatula to frost the top and sides of the chilled cake with the remaining frosting. Pipe a shell border along the top and bottom edges using the reserved frosting in the piping bag. Add sprinkles.

Make-ahead planning and storage

This cake is designed for parties, so it's worth knowing exactly how far ahead each piece can go.

Cake layers: Bake up to one day ahead. Wrap completely cooled layers in plastic wrap and store at room temperature, per Delish. For longer lead times, freeze tightly wrapped layers for up to one month (Delish) or up to three months per The Slow Roasted Italian. Thaw at room temperature before assembly.

Frosting: Make up to three days ahead and refrigerate in an airtight container. Re-beat in the stand mixer before using it stiffens in the fridge. The all-shortening version re-whips easily, per Delish.

Mousse: Day of assembly only.

Assembled cake: Delish puts the window at up to three days at room temperature or up to one week refrigerated. Given the cream-cheese filling, The Slow Roasted Italian is more conservative at three days refrigerated. Refrigerate after day one regardless.

For context: ordering a custom Costco cake requires at least 24 hours' notice, per Delish. With the make-ahead structure above, this version fits comfortably in the same window and the decoration is entirely yours.

When something goes wrong: four common problems

Mousse too loose. Usually means too much milk or the pudding packet wasn't added dry. The box directions call for more liquid than this recipe uses ignore them completely. If the mousse won't hold its shape, refrigerate it for 15 minutes before spreading.

Frosting too stiff. The all-shortening frosting can firm up significantly, especially if the room is cool. Add water one teaspoon at a time and beat on medium until it loosens. If it was made ahead and refrigerated, re-beat it fully in the stand mixer before trying to adjust the texture.

Layers sliding during assembly. Skip the mousse-to-edge approach if you're having trouble. Spread to a half-inch border as described in Step 8, chill the mousse layer for the full 15 minutes before adding the second cake, and refrigerate the assembled stack again before applying the outer frosting. Rushing either chill step is what causes movement.

Cake overbrowning or doming. The doctored batter bakes faster than a standard scratch cake. Start checking at 20 minutes, not 25. If the edges are browning faster than the center, tent loosely with foil for the last few minutes. Some doming is normal and gets trimmed away in Step 5 don't overbake trying to flatten it.

Which version to make

The all-shortening path here is the right call when you want the thing to taste like a Costco cake. That's the point. Box mix, instant pudding, and Crisco aren't workarounds for this particular cake they're what produces the specific texture and flavor people recognize.

If you're serving a crowd that finds the shortening frosting too sweet, or if authenticity matters less than richness, the half-butter hybrid from 12 Tomatoes gives you more flavor complexity without abandoning the bakery character entirely. The all-butter, from-scratch approach at The Slow Roasted Italian is genuinely good cake it just isn't a Costco copycat in any meaningful sense.

For a party where someone specifically asked for the Costco cake: shortening frosting, doctored mix, instant pudding. That's the recipe.