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Marble Sheet Cake

Ingredients

For the cake:

  • 1/2 cup boiling water
  • 1/2 cup dark cocoa powder
  • 1 1/2 cups softened butter
  • 1 1/2 cups sugar
  • 6 eggs
  • 1/2 cup mayonnaise
  • 2 tsp. vanilla extract
  • 3 cups flour
  • 4 1/4 tsp. baking powder
  • 1 tsp. salt
  • 1/4 cup milk

For the frosting:

  • 1/3 cup chocolate chips, melted and cooled
  • 1 cup softened butter
  • 2 cups powdered sugar
  • 1/4 tsp. salt
  • 1/2 tsp. vanilla extract
  • 1 tbs. dark chocolate cocoa powder
  • 1/4 cup heavy cream

Instructions

For the cake:

  1. Preheat your oven to 350 degrees, and line a half-sheet pan with parchment paper. Lightly spray the pan with cooking spray. Set aside. 

  2. In a small bowl, whisk together the boiling water and cocoa powder until a smooth, thick paste is formed. Set aside. 

  3. In a large bowl, beat together the butter and sugar until fluffy. Add in the eggs one at a time, mixing between each addition. Beat in the mayonnaise and vanilla. 

  4. Add in the flour, baking powder, and salt, and beat until combined. Then, beat in the milk and mix until the batter is smooth- it will be fairly thick. 

  5. Pour about 1/3 of the batter into a medium bowl- it does not have to be exact. Beat in the cocoa powder-water mixture to the 1/3 batter until completely combined. 

  6. Scoop the vanilla batter onto your sheet pan with an ice cream scoop, leaving spaces for the chocolate batter. Then, scoop the chocolate batter onto the pan as well. Using a spatula, spread the batter to the edges of the pan. Then, use a butter knife to swirl the batters together. 

  7. Bake for 20 minutes, or until a cake tester comes out clean. Cool completely before frosting. 

For the frosting:

  1. Beat together the butter, powdered sugar, salt, and vanilla until smooth. Beat in the melted chocolate, and then the heavy cream until the mixture is fluffy. Spread evenly onto the cooled cake. There won't be much excess here, so you'll need to spread it thin. Enjoy!