Copycat Costco Sheet Cake

Copycat Costco Sheet Cake is the answer for every celebration when you want that iconic fluffy texture with creamy mousse filling and the nostalgic bakery frosting everyone remembers from birthday parties. This recipe brings all the grandeur and flavor of the original right into your kitchen without a trip to the store.

The first time I made this was for my daughter’s graduation party and it disappeared so fast even before I could grab a second slice. Now it is on repeat for every family milestone.

Ingredients

• White cake mix: gives you the iconic light crumb and you can find reliable brands in any grocery store
• Kosher salt: enhances all the flavors and cuts sweetness
• Large egg whites: make the cake extra airy and help it stay snow white
• Sour cream: adds moisture for that signature soft bite
• Whole milk: brings a richer taste than water alone
• Vegetable oil: keeps the crumb tender and moist
• Pure vanilla extract: opt for a clear variety if you want that true bakery-white color
• Cream cheese: forms the base of the mousse filling giving tang and creaminess
• Confectioners sugar: sweetens gently and keeps the frosting fluffy
• Vanilla instant pudding mix: ensures the mousse is silky and stable
• Heavy cream: whips up light and billowy for both the mousse and frosting
• Vegetable shortening: is classic for old-school bakery frosting and helps hold shape in piping
• Sprinkles: add festive color and you can use classic rainbow or tailor to your celebration

• Tip when choosing cake mix: check the label for a pale white batter since some mixes contain yellow pigments

Step-by-Step Instructions

Prepare the Baking Pans:

Line two twelve by nine inch metal baking pans with parchment paper and coat lightly with cooking spray to make for easy cake release later

Mix the Cake Batter:

Combine the cake mixes and kosher salt in your stand mixer. Add egg whites sour cream milk oil and vanilla. Beat on medium speed for about two minutes until the batter is smooth and thick

Bake the Cakes:

Divide batter evenly between your pans. Bake at three hundred fifty degrees Fahrenheit for twenty to twenty five minutes. You want the tops lightly browned yet springy when pressed. Cool the cakes in pans for ten minutes then invert them onto a rack. Let them cool completely. If needed trim any domed tops for even layering

Make the Mousse Filling:

In a clean mixer bowl beat cream cheese confectioners sugar and salt until completely smooth about one to two minutes. Add whole milk and dry vanilla pudding mix and continue mixing for another minute. Pour in heavy cream and beat until the mousse is light thick and fluffy about one to two more minutes

Assemble the Layers:

Place one cake layer onto your serving platter. Spread the mousse filling evenly right to the edges but leave a half inch border so it does not ooze out. Place the second layer on top with the trimmed side down. Chill the stacked cake to set the mousse

Mix the Frosting:

Beat together the vegetable shortening water vanilla and salt until creamy. Gradually add the confectioners sugar one cup at a time mixing on low to avoid mess then increase to medium high and beat for one minute until fluffy

Decorate the Cake:

Using an offset spatula frost the cake with a thick even layer. Fill a piping bag fitted with a star tip with some frosting and pipe shell borders around the edges. Sprinkle on your favorite festive sprinkles

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My favorite part is always the mousse filling. When I was a kid my mom would let me lick the bowl after she finished building the cakes. That tangy sweet flavor stuck with me so now I double the mousse whenever I make extra for the family.

Storage Tips

This cake holds up well covered in the fridge for four to five days and stays soft. I like to chill it completely before slicing for cleaner cake layers. For longer storage wrap individual slices tightly and freeze up to two months. Let thaw in the fridge overnight for best texture.

Ingredient Substitutions

If you prefer a yellow cake you can use yellow mix but it will change the flavor. For the mousse filling swap cheesecake pudding for vanilla for a different twist. If you want a less sweet frosting try using half butter half shortening though pure shortening is essential for that true bakery style stability. Lactose intolerant guests can use dairy free cream cheese and milk.

Serving Suggestions

After adding sprinkles you can also top with fresh berries or piped rosettes for special occasions. Serve chilled for the cleanest slices or let it come to room temperature if you like a softer feel. Pair with coffee or cold milk and your party will have everyone coming back for seconds

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Cultural Context

The legendary Costco sheet cake became famous for feeding huge crowds at a friendly budget so it is a beloved staple for birthdays anniversaries and big family gatherings. Its light fluffy crumb and sweet smooth frosting are tied to nostalgic memories and big celebrations across the country.

Recipe FAQs

→ How do I ensure the cake layers stay moist?

Use ingredients like sour cream and egg whites, and avoid overbaking. Check for doneness at the minimum baking time.

→ Can I use a different flavor of cake mix?

Yes, yellow or chocolate cake mix works well, but texture and flavor will vary from the classic white base.

→ What’s the secret to fluffy frosting?

Beat the shortening and confectioners’ sugar until light and airy. Gradually add sugar and keep mixing until fluffy.

→ How far in advance can I assemble the cake?

Assemble and chill the finished cake up to 1 day ahead. Frosting and decorations hold best when added close to serving.

→ Is it possible to make this dessert without a stand mixer?

An electric hand mixer works well for all mixing steps, though a stand mixer simplifies the process for larger batches.

→ How can I achieve clean, professional-looking slices?

Chill the assembled cake before cutting and use a sharp, warmed knife, wiping between slices for best results.