Copycat Costco Sheet Cake

Nothing says party like a giant bakery sheet cake covered in sprinkles, and this copycat Costco Sheet Cake is no exception. Layers of fluffy white cake are filled with vanilla cheesecake mousse and covered with light and airy frosting for a homemade version that tastes just like OG—no membership required.

Creating a copycat version of any mass-produced item with easy-to-find ingredients is always a fun challenge. To ensure this cake was as close to the original as possible, I ordered a Costco cake and tasted each of its components separately and together. Here’s what I learned about each—and the surprise discoveries I had along the way.

The cake:
Many copycat versions of this cake include recipes for a homemade white cake, but if you taste the actual Costco cake, it tastes super similar to a boxed mix. Which makes sense—do you think they’re making their cakes completely from scratch? I did notice, however, that the Costco cake was slightly denser than a typical cake made with a boxed mix, which has a fluffier, “looser” crumb—and this led me to a doctored box mix white cake. If you’ve never made one before, a doctored boxed mix cake is a cake that uses a boxed cake mix for its dry ingredients but includes different amounts or types of wet ingredients and additional flavor enhancers. Substituting milk and sour cream for water (and some of the oil) leads to that slightly denser crumb structure here, while a little salt and vanilla extract really makes those flavors pop.

The vanilla cheesecake mousse:
Costco’s vanilla sheet cake comes filled with a “vanilla cheesecake mousse”—which, as it turns out, is just a combination of cream cheese and instant vanilla pudding with some additional confectioners’ sugar and heavy cream added to lighten things up. Make sure you’re using instant (not stovetop) vanilla pudding here, and pour it directly from the pouch into the mixing bowl with 1 cup milk specified in the recipe. Ignore the instructions on the back of the box.

The frosting:
As much as I love an all-butter buttercream made with a little heavy cream, I have to admit: the right frosting in this case is made with straight-up Crisco mixed with water and confectioners’ sugar. This shocked me—and most everyone else in the test kitchen—but everyone who tasted it agreed: It tastes just like that bakery frosting that we all know and love. So while it goes against most of my buttercream principles, in it goes!

Make ahead & storage:
The cakes can be made up to 1 day ahead. Let cool completely, then wrap in plastic wrap and store at room temperature. Or, if you really want to work ahead, you can wrap in plastic wrap and freeze for up to 1 month. Thaw at room temperature, then proceed to the assembly.

While the mousse is best used to fill the cake immediately after it’s made, the frosting can also be made up to 3 days ahead. Store in an airtight container in the refrigerator and beat in a stand mixer to loosen before using to frost the cake.

Once assembled, the cake will stay good for up to 3 days at room temperature or up to 1 week in the refrigerator.

Made this recipe? Let me know how it went in the comments below!