Chocolate Sheet Cake with Double Chocolate Mousse Frosting

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This Chocolate Sheet Cake is rich and moist, with a deep chocolate flavor that satisfies any sweet tooth. It has a beautifully light and tender crumb. The double chocolate mousse frosting is smooth and creamy, adding a delicious layer to each bite. Finished with a sprinkle of colorful sprinkles, this cake looks as good as it tastes.

Table of Contents

What is a Sheet Cake?

A sheet cake is a large, flat cake baked in a rectangular pan, usually 9x13 inches. It's great for serving at parties because you can cut it into many pieces. Sheet cakes are simple to make and can be decorated with frosting and sprinkles for any occasion.

Ingredients for Chocolate Sheet Cake

All-Purpose Flour: Provides structure to the cake. You can also use our homemade Cake Flour.
Cocoa Powder: Adds rich chocolate flavor.
Baking Soda: Helps the cake rise.
Vegetable Oil: Keeps the cake moist.
White Sugar: Sweetens the cake and adds moisture.
Brown Sugar: Adds a deeper flavor and extra moisture.
Large Eggs: Bind the ingredients and add richness.
Vanilla Extract: Enhances the flavor.
Buttermilk: Adds moisture and a slight tang, which reacts with the baking soda for a tender crumb.

Double Chocolate Mousse Frosting:

Cocoa Powder: Adds a deep chocolate flavor.
Hot Water: Blooms the cocoa powder to enhance its flavor.
Dark or Milk Chocolate: Adds richness and depth to the frosting.
Heavy Cream: Makes the frosting smooth and creamy.
Softened Unsalted Butter: Provides a creamy base and helps create a fluffy texture.
Confectioner's Sugar: Sweetens and thickens the frosting.

Garnish:

Sprinkles: Add a fun and colorful touch to the finished cake.

How To Make Chocolate Sheet Cake

Preparing the Cake Batter

Preheat and Prepare Pan: Preheat your oven to 350°F (175°C). Grease and line a 9x13-inch pan with parchment paper.

Mix Dry Ingredients: Sift together the all-purpose flour, cocoa powder, and baking soda into a large bowl. Mix well with a hand whisk to ensure even distribution.

Combine Wet Ingredients: In a separate bowl, combine the white sugar, brown sugar, vegetable oil, eggs, vanilla extract, and buttermilk. Whisk until the mixture is well combined.

Combine Wet and Dry Ingredients: Pour the wet mixture into the bowl with the dry ingredients. Mix with a silicone spatula until the flour is just incorporated, then switch to a hand whisk and mix until the batter is smooth and glossy.

Pour Batter into Pan: Pour the batter into the prepared pan, spreading it evenly.

Baking the Chocolate Sheet Cake

Bake the Cake: Place the pan in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Cool the Cake: Remove the cake from the oven and let it cool in the pan on a wire rack for about 10 minutes. Then, turn the cake out onto the wire rack to cool completely before frosting.

Making the Double Chocolate Mousse Frosting

Prepare Cocoa Paste: Combine the cocoa powder and hot water in a small bowl, mixing well to form a paste.

Melt Chocolate and Cream: In a separate heatproof bowl, add the chopped chocolate and heavy cream. Microwave in 20-second intervals, stirring each time until smooth. Let cool to room temperature.

Mix Butter and Sugar: In another large bowl, beat the softened unsalted butter and powdered sugar with a hand mixer until light and fluffy.

Combine Cocoa Paste with Butter Mixture: Add the cooled cocoa powder mixture to the butter mixture and mix until well combined.

Add Chocolate Mixture: Finally, add the room temperature chocolate and cream mixture to the frosting, mixing carefully to avoid overmixing.

Frosting the Chocolate Sheet Cake

Frost the Cake: Spread the double chocolate mousse frosting evenly over the cooled cake using an angled spatula, ensuring it covers the entire surface.

Add Garnish: Sprinkle the top with colorful sprinkles.

Refrigerate: Refrigerate the cake until ready to serve. This chocolate sheet cake can be made and frosted a day in advance; just keep it covered in the refrigerator to prevent it from drying out.

Fay's Top Tip

An important tip is to avoid overmixing the frosting after adding the chocolate mixture. Overmixing can cause the frosting to curdle, affecting its texture and appearance. Mix just until everything is combined and smooth.

Recipe Variations

Chocolate Chip Addition: Add 1 cup (150g) of chocolate chips to the cake batter for an extra chocolatey surprise.
Nutty Delight: Stir in 1 cup (120g) of chopped nuts like walnuts or pecans into the batter for added crunch and flavor.
Berry Boost: Fold in 1 cup (150g) of fresh or frozen raspberries or strawberries to the batter for a fruity twist.
Coffee Kick: Add 1 tablespoon of instant coffee granules to the hot water used in the frosting for a mocha flavor.
Coconut Twist: Mix 1 cup (100g) of shredded coconut into the frosting for a tropical flavor.
Alternate Frosting: Replace the double chocolate mousse frosting with a classic vanilla buttercream, cream cheese frosting, or choose from the many buttercreams and frostings we have on the blog for a different flavor profile.

Serving Suggestions

With Ice Cream: Serve a slice of the cake with a scoop of vanilla ice cream or coffee ice cream on the side.
Drizzle with Sauce: Drizzle caramel sauce or chocolate hot fudge sauce over each slice for added richness.
Fresh Berries: Garnish the cake with fresh berries like strawberries, raspberries, or blueberries for a burst of color and flavor.
Whipped Cream: Add a dollop of whipped cream on top of each slice for an extra creamy touch.
Warm and Gooey: Warm the cake slightly in the microwave before serving for a gooey, melt-in-your-mouth experience.

More Buttercream and Frosting Ideas

See more Buttercream and Frostings →

Common Questions

Can I use a different type of flour?

You can use cake flour for a lighter texture or whole wheat flour for a denser, healthier option. Adjust the liquid as needed.

How can I make this cake gluten-free?

Substitute the all-purpose flour with a gluten-free flour blend. Ensure all other ingredients are gluten-free as well.

Can I make this chocolate sheet cake ahead of time?

Yes, you can bake the cake a day ahead and store it covered in the refrigerator. Frost it on the day you plan to serve it.

What if I don't have buttermilk?

You can make a substitute by adding 1 tablespoon of lemon juice or vinegar to 1 cup of milk. Let it sit for 5 minutes before using.

Can I freeze the cake?

Yes, you can freeze the unfrosted cake. Wrap it tightly in plastic wrap and foil, then freeze for up to 3 months. Thaw before frosting and serving.

What can I use instead of vegetable oil?

You can use melted butter, coconut oil, or another neutral-flavored oil as a substitute for vegetable oil.

Can I add flavor extracts to the frosting?

Yes, you can add extracts like almond, peppermint, or orange to the frosting for a different flavor profile. Start with a small amount and adjust to taste.