Cosmic Brownie Sheet Cake

Top view of the cake pan with the chocolate cake inside

If you grew up eating the nostalgic Little Debbie Cosmic Brownies then you are going to love this cake recipe for Cosmic Brownie Sheet Cake. This rich, ultra-moist chocolate cake starts with a doctored up cake mix, topped with a silky smooth chocolate ganache and finished with those iconic rainbow candy-coated chocolate pieces.

Course Cake, Dessert

Cuisine American

Keyword cosmic brownie sheet cake, little debbie copycat cosmic brownie cake

Prep Time 20

Cook Time 35

Cooling Time 1 45

Total Time 2 40

Servings

Calories 287kcal

Author Jessica - Together as Family

Equipment

Ingredients

Chocolate Fudge Cake

1 box (13.25 oz) chocolate fudge cake mix
4 large eggs
1 cup whole milk
1/2 cup vegetable oil
1/4 cup full fat sour cream
2 teaspoons pure vanilla extract

Chocolate Ganache Frosting

1 cup heavy cream
2½ cups semi-sweet chocolate chips
2 tablespoons salted butter ^{(if using unsalted butter add ¼ teaspoon kosher salt)}
1 teaspoon pure vanilla extract
rainbow cosmic candy chips

Instructions

Preheat the oven to 350°F. Prepare a 9x13-inch cake pan by spraying with nonstick cooking spray. Set it aside.
In a large bowl, add the dry chocolate fudge cake mix, large eggs, whole milk, vegetable oil, sour cream, and pure vanilla extract. Use a hand or stand mixer, to beat on low speed on 30 seconds. Increase the speed to medium-high and continue beating for 2 minutes, scraping the bottom and sides of the bowl as needed.
1 box (13.25 oz) chocolate fudge cake mix, 4 large eggs, 1 cup whole milk, 1/2 cup vegetable oil, 1/4 cup full fat sour cream, 2 teaspoons pure vanilla extract
Pour the batter into the prepared cake pan and spread it into an even layer. Bake for 23-28 minutes, or until the center springs back when gently touched and a toothpick inserted into the center comes out clean.
Remove the cake from the oven and place it on a wire cooling rack and allow the cake to cool completely, in the pan for 1 hour.
In a small saucepan, over medium-low heat, add the heavy cream and heat it until it begins to steam and small bubbles form around the edges. Do not allow the cream to boil and make sure you are stirring it frequently.
1 cup heavy cream
Add the semi-sweet chocolate chips to a medium sized heat-safe bowl (like a glass bowl). Pour the hot cream over the chocolate chips, making sure the chocolate is mostly covered. Let the mixture sit undisturbed for 3 minutes.
2½ cups semi-sweet chocolate chips
Add the softened butter and pure vanilla extract into the chocolate mixture. Whisk slowly from the center outward until the chocolate is completely melted and the ganache is smooth, glossy, and evenly combined.
2 tablespoons salted butter, 1 teaspoon pure vanilla extract
Let the ganache sit at room temperature for about 10 minutes, stirring occasionally, until it thickens slightly but is still easy to spread.
Pour the chocolate ganache over the completely cooled cake and use an offset spatula to spread it out evenly to the edges. Immediately sprinkle the rainbow chips over top.
rainbow cosmic candy chips
Allow the cake to stand at room temperature for about 30 minutes to 1 hour, to give the ganache time to set, before slicing and serving. Enjoy!

Notes

Make sure the cake is completely cooled before adding the ganache. Even a slightly warm cake can cause the ganache to melt, soak into the surface, or sliced toward the edges.
Use whole milk and full-fat sour cream for the best and richest texture and taste in the cake. This recipe uses a semi-homemade, 'doctored up cake mix' hack to make it taste even better.
Only warm the heavy cream just until it begins to steam, and small bubbles form around the edges. Boiling the cream can cause too much moisture to evaporate and may make the finished ganache greasy or overly thick. Also, make sure you are stirring the cream very frequently to also avoid it from scalding.
Letting the ganache cool slightly before pouring it over the cake is important. It should pour easily but should not be so warm and thin still that it runs off the sides of the cake or each slice. The ganache should form a thick layer on it's own and stay there. If the ganache seems too thin, let it continue sitting out at room temperature to cool longer.
Look for the rainbow cosmic chips either in the baking aisle near the sprinkles, the cake decorating aisle (like at a Walmart), or you can sometimes also find them near the ice cream sundae toppings.
I use salted butter in the ganache recipe, but if you are using unsalted butter, make sure to add ¼ teaspoon kosher salt in with the butter.

Nutrition