Copycat Costco Sheet Cake Recipe

Name: Copycat Costco Sheet Cake Recipe
Prep time: 25 min
Cook time: 35 min
Total time: 60 min
Yield: 20
Cuisine: American
Ingredients: 2 cups all-purpose flour; 2 cups granulated sugar; 1 cup unsalted butter, softened; 1 cup whole milk; 4 large eggs; 3/4 cup cocoa powder; 1 teaspoon salt; 1 1/2 teaspoons baking soda; 1 cup hot coffee; 1 tablespoon vanilla extract; 1/2 cup butter (for frosting); 1/3 cup milk (for frosting); 1/4 cup cocoa powder (for frosting); 4 cups powdered sugar (for frosting); 1 teaspoon vanilla extract (for frosting)

I’ll never forget the first time I tried to make a Copycat Costco Sheet Cake. It was for my nephew’s birthday, and I was so confident. I mean, how hard could it be? Well, I learned. The cake came out dense as a brick and the frosting was a grainy, sugary mess. My brother-in-law, trying to be nice, said it had “great structural integrity.” We ended up eating it with ice cream to soften it up, and honestly, it was still a pretty good time. That failure lit a fire under me, and after a lot of trial and error, I finally figured out how to nail that fluffy, moist cake and that signature fudgy frosting.

Recipe Card

Recipe Title	Copycat Costco Sheet Cake Recipe
Servings	20
Prep Time	25 minutes
Cooking Time	30-35 minutes
Calories	Approx. 480 per slice

Ingredients

2 cups all-purpose flour
2 cups granulated sugar
1 cup unsalted butter, softened
1 cup whole milk
4 large eggs
3/4 cup cocoa powder
1 teaspoon salt
1 1/2 teaspoons baking soda
1 cup hot coffee
1 tablespoon vanilla extract
For the Frosting: 1/2 cup butter, 1/3 cup milk, 1/4 cup cocoa powder, 4 cups powdered sugar, 1 tsp vanilla

That hot coffee is the secret weapon. The first time I saw it in a recipe I was like, “nope, that can’t be right.” But it doesn’t make it taste like coffee, I promise. It just makes the chocolate flavor so much deeper and richer. I tried skipping it once and the cake was just…sad. Flat. Don’t be scared of the coffee.

Directions

Preheat your oven to 350°F. Grease a 10×15 inch jelly roll pan. Don’t use a 9×13, it will overflow. I learned that the messy way.
In a big bowl, cream together the softened butter and sugar until it’s light and fluffy. This takes a few minutes, don’t rush it.
Beat in the eggs one at a time, then stir in the vanilla.
In a separate bowl, whisk together the flour, cocoa powder, salt, and baking soda.
Add the dry ingredients to the butter mixture alternately with the milk, starting and ending with the flour mix. Stir until just combined.
Carefully stir in the hot coffee. The batter will be very thin, that’s totally normal! Don’t panic.
Pour the thin batter into your prepared pan and spread it evenly.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely in the pan on a wire rack. I know it’s hard to wait. But you have to.
For the frosting, melt the 1/2 cup butter in a saucepan over medium heat.
Whisk in the milk and cocoa powder until it’s smooth.
Bring it to a gentle boil, then immediately remove it from the heat.
Whisk in the powdered sugar and vanilla until it’s smooth and pourable.
Pour the warm frosting over the completely cooled cake and spread it quickly before it sets.

The biggest lesson I learned was with the frosting. I got impatient and poured it onto a warm cake once. It turned into a melty, gloppy puddle that soaked into the cake and made the whole thing way too sweet. It was a sugar bomb. Letting the cake cool is the most boring but most important step. Go fold some laundry or something.

What I love about this Copycat Costco Sheet Cake is that it’s a total crowd-pleaser. It’s the cake I bring to every potluck now, and people always ask for the recipe. It’s my go-to easy dessert for a big family gathering. The leftovers, if you have any, are actually amazing the next day. The frosting gets a little firmer and it’s just perfect with a glass of cold milk.

I did have one major disaster where I mixed up baking soda and baking powder. I was distracted and just grabbed the wrong container. The cake tasted weirdly metallic and didn’t rise at all. It was a complete waste of ingredients. Now I always double-check the labels before I start. It’s a simple mistake but it ruins everything.

If I were to change one thing next time, I might try adding a tiny pinch of espresso powder to the frosting to really amp up the chocolate flavor even more. I’ve heard that works wonders, and I’m all for making a good thing even better. It’s such a simple comfort food, but it feels so special.

Nutrition Info (per serving)

Calories	Carbs	Fat	Protein
480	72g	21g	5g

Okay, let’s be real, this is a sheet cake. It’s not a health food. The numbers are what they are. I’ve tried making it with lower-fat milk and it was fine, but the texture of the cake wasn’t quite as tender. For a special treat, I say go for the whole milk and butter. If you’re watching sugar, you could maybe reduce the sugar in the cake a tiny bit, but the frosting is non-negotiable for that authentic taste.

Ingredient Swaps

Ingredient	Substitution
Whole Milk	2% or Almond Milk
Hot Coffee	Hot Water
Buttermilk	Milk + 1 tbsp vinegar

I’ve tried the buttermilk swap and it gives the cake a nice little tang, but it’s not the same as the original. Using hot water instead of coffee works in a pinch, but you really do lose that deep chocolate flavor. It just tastes…lighter. Almond milk works okay, but the cake can be a bit more crumbly. My advice? Stick to the recipe if you can, but don’t let a missing ingredient stop you from baking.

Tips

Use a kitchen scale to measure your flour. Scooping with a cup can pack in too much and lead to a dry cake.
Don’t overmix the batter after adding the flour. Just stir until the streaks of flour disappear.
For a super smooth frosting, sift your powdered sugar and cocoa powder. It’s a pain but it prevents lumps.
Use a toothpick to check for doneness. The top might look done before the middle is actually cooked.

I learned the overmixing lesson the hard way. I kept beating the batter thinking I was making it extra smooth, and I ended up with a tough, rubbery cake. It was like eating a chocolate sponge. Now I’m so gentle with it, just a few folds and I’m done. It feels wrong, but it makes the cake so much softer.

FAQ

Can I make this in a 9×13 pan?
You can, but it will be a much thicker cake and the baking time will be longer, maybe 40-50 minutes. You also won’t get that same thin, classic sheet cake texture. I did it once and the edges were a bit overdone by the time the middle was cooked. A jelly roll pan is best.

My frosting got lumpy! What do I do?
Oh, I’ve been there. If your powdered sugar is lumpy, your frosting will be lumpy. The best fix is to push the warm frosting through a fine-mesh sieve. It’s a bit messy, but it saves the whole thing. Now I just sift my sugar and cocoa right from the start.

Why is my cake dry?
The two most likely culprits are overbaking or over-measuring the flour. Ovens can run hot, so start checking at 25 minutes. And please, fluff your flour with a spoon before you scoop it into the measuring cup. It makes a huge difference.

That’s everything I know about making this Copycat Costco Sheet Cake! Hope you give it a try. And if you mess it up, you’re in good company—I’ve definitely had my share of kitchen fails with this one. Happy baking!