If you ever had a slice of Costco’s famous chocolate sheet cake, you know it was the dessert that disappeared first at birthdays, potlucks, and office parties. Moist, rich, and smothered in silky chocolate frosting, it was a crowd favorite for good reason.
While it’s no longer regularly sold in stores, this Costco Chocolate Cake copycat recipe brings all that bakery magic back to life — and honestly, it might be even better made from scratch.
I’ve tested this recipe over and over to match the iconic flavor and texture — the fudgy crumb, deep chocolate taste, and that soft whipped frosting. It’s a showstopper of a cake that feeds a crowd and earns rave reviews every time.
Whether you’re recreating a childhood favorite or just love rich chocolate desserts, this one’s a must-bake.
Why You’ll Love This Recipe
This cake is everything a chocolate lover dreams of — moist, dense, and packed with bold cocoa flavor. But unlike some overly rich cakes, this one has a smooth, soft crumb that melts in your mouth and holds up beautifully to thick layers of frosting.
The frosting is the real star: fluffy, chocolatey, and not too sweet. It spreads like a dream and turns the whole cake into a bakery-style centerpiece perfect for birthdays, celebrations, or a just-because dessert.
What You’ll Need
For the Cake:
- 2 cups all-purpose flour
- 2 cups granulated sugar
- ¾ cup unsweetened cocoa powder
- 2 tsp baking powder
- 1 ½ tsp baking soda
- 1 tsp salt
- 1 cup buttermilk (room temperature)
- ½ cup vegetable oil
- 2 large eggs (room temperature)
- 2 tsp vanilla extract
- 1 cup hot water or brewed coffee (for richness)
For the Frosting:
- 1 cup (2 sticks) unsalted butter, softened
- ½ cup unsweetened cocoa powder
- 3–3½ cups powdered sugar
- ¼ cup heavy cream (plus more if needed)
- 1 tsp vanilla extract
- Pinch of salt
- Optional: chocolate curls or sprinkles for topping
Pro Tips
- Use hot coffee instead of water — it deepens the chocolate flavor without making the cake taste like coffee.
- Don’t overmix the batter — stir just until combined for a tender crumb.
- Let the cake cool completely before frosting — otherwise, the buttercream will melt.
- Sift cocoa powder and powdered sugar for extra-smooth frosting.
- Want that Costco “look”? Add piped swirls of frosting and chocolate shavings on top.
Tools You’ll Need
- Mixing bowls
- Stand or hand mixer
- Whisk and spatula
- Measuring cups and spoons
- 9×13 sheet cake pan
- Cooling rack
- Offset spatula (for frosting)
Substitutions & Variations
- No buttermilk? Use 1 cup milk + 1 tbsp vinegar or lemon juice.
- Dairy-free version: Use dairy-free milk, plant-based butter, and vegan-friendly frosting alternatives.
- Make it a layer cake: Divide the batter between two 9-inch round pans and reduce baking time to 28–32 minutes.
- Add chocolate chips to the batter for extra gooey bites.
Make Ahead Tips
You can bake the cake a day ahead, wrap tightly, and frost the next day. The frosting also keeps well in the fridge — just bring it to room temperature and re-whip before spreading.
The finished cake can be stored for 2–3 days at room temperature or up to a week in the fridge.
Instructions
1. Preheat & Prep
Preheat your oven to 350°F (175°C). Grease a 9×13-inch pan and line with parchment paper if desired for easy removal.
2. Mix Dry Ingredients
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
3. Add Wet Ingredients
Add buttermilk, oil, eggs, and vanilla to the dry ingredients. Beat until just combined. Slowly add hot coffee or water and mix until smooth.
4. Bake
Pour batter into prepared pan and smooth the top. Bake for 35–40 minutes, or until a toothpick comes out clean. Cool completely on a wire rack.
5. Make the Frosting
In a large bowl, beat softened butter until fluffy. Add cocoa powder, powdered sugar, vanilla, and salt. Gradually beat in cream until smooth and fluffy.
6. Frost & Decorate
Spread frosting evenly over the cooled cake. Add decorative swirls, sprinkles, or shaved chocolate if desired.
Leftovers & Storage
Store the frosted cake covered at room temperature for up to 3 days, or refrigerate for up to 7 days. Let come to room temp before serving.
You can also freeze unfrosted cake layers for up to 2 months — wrap tightly in plastic and foil.
Recipe FAQs
1. Can I use Dutch-process cocoa powder?
Yes — but it may affect the rise slightly. Natural cocoa is best for this recipe, but both work well for flavor.
2. What makes Costco cake so moist?
A combination of buttermilk, oil, and hot water (or coffee) keeps it super tender and rich.
3. Can I make cupcakes instead?
Absolutely! Bake at 350°F for 18–20 minutes. It makes about 24 cupcakes.
4. Can I halve this recipe?
Yes — just use an 8×8 pan and bake for 30–35 minutes.
The Final Slice
This Costco Chocolate Cake copycat recipe is rich, moist, and unapologetically chocolatey — just like the bakery classic you remember.
Whether you’re baking it for a crowd, a birthday, or your own sweet tooth, it’s a guaranteed hit. Try it once and you might just ditch the store-bought version forever.
If you bake it, drop a comment and tell me what occasion you made it for — and how fast it disappeared!