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Cosmic Brownie Sheet Cake

This Cosmic Brownie Sheet Cake is a fun, nostalgic twist on the classic lunchbox treat! It features a moist, brownie-like chocolate cake topped with a smooth bittersweet ganache and those signature rainbow chips. It’s rich, sliceable, and perfect for parties, birthdays, or anytime you want a homemade upgrade of a childhood favorite.
Prep Time20 minutes
Cook Time30 minutes
Servings: 12 slices
Author: Emily - My Messy Kitchen

Ingredients

For the cake:

  • 1 cup salted butter melted
  • ½ cup vegetable oil
  • 1 ¾ cup granulated sugar
  • ¾ cup light brown sugar packed
  • 4 eggs room temperature
  • 1 tsp vanilla extract
  • 1 cup sour cream room temperature
  • 1 cup hot water you can also use coffee!
  • ¾ cup unsweetened cocoa powder
  • 2 cups all-purpose flour
  • 1 tsp baking soda
  • ½ tsp baking powder
  • ½ tsp salt

For the ganache:

  • 8 oz bittersweet chocolate finely chopped
  • ¾ cup heavy cream
  • ½ cup rainbow chips

Instructions

For the cake:

  • Preheat oven to 350°F. Grease and line a 9x13-inch pan with parchment paper. In a large bowl, whisk together the melted butter, oil, granulated sugar, and brown sugar until smooth. Add eggs one at a time, whisking well after each. Stir in the vanilla and sour cream.
  • In a separate bowl, whisk the cocoa powder into the hot water until fully dissolved. Add to the batter and mix to combine. Sift in the flour, baking soda, baking powder, and salt. Stir until just combined—do not overmix.
  • Pour the batter into the prepared pan and smooth out the top. Bake for 30–35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Let the cake cool completely in the pan.

For the ganache:

  • Heat the heavy cream in a small saucepan (or microwave-safe bowl) until just simmering—don’t boil. Pour the hot cream over the chopped chocolate. Let sit for 1–2 minutes to soften, then whisk gently until smooth and glossy. If not completely melted, microwave for 20 seconds at a time, whisking in between, until smooth.
  • Let the ganache cool for 10–15 minutes, until slightly thickened but still pourable. It should spread easily but not run.

For assembly:

  • Pour the ganache over the fully cooled cake and spread it evenly to the edges with an offset spatula or spoon. Immediately top with rainbow chips while the ganache is still soft. Allow it to set for 20–30 minutes to help the ganache set to a sliceable, fudgy consistency before slicing!