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3.92 from 12 votes

Chocolate Rainbow Cake

Prep Time3 hours
Cook Time30 minutes
Servings: 12

Ingredients

Chocolate Cake

  • 1 cup (85 g) unsweetened cocoa powder
  • 1 cup (240 ml) freshly brewed hot coffee or hot water
  • 1 ¾ cups (245 g) all-purpose flour
  • 1 ½ cups (300 g) granulated sugar
  • 1 ½ teaspoons baking soda
  • 1 ½ teaspoons baking powder
  • ½ teaspoon salt
  • 2 eggs room temperature
  • 1 cup (240 ml) buttermilk
  • ½ cup (113 g) butter melted
  • 2 teaspoons vanilla extract

Chocolate Ganache Frosting

  • 1 cup (250 ml) heavy cream
  • ½ cup (113 g) butter
  • ⅓ cup (70 g) brown sugar
  • ¼ teaspoon salt
  • 9 ounces (250 g) semi-sweet baking chocolate chopped or chocolate chips
  • 7 ounces (200 g) 70% dark chocolate chopped
  • 1 teaspoon vanilla extract

Vanilla Rainbow Frosting

  • 1 cup plus 2 tablespoons (250 g) butter at room temperature
  • A pinch of salt
  • 3 ½ cups (400 g) powdered sugar sifted
  • 1 tablespoon vanilla extract
  • ¼ cup (60 ml) heavy cream
  • Food coloring

Instructions

Chocolate Cake

  • Preheat oven to 350°F (175°C). Grease and flour two 8-inch (20 cm) round cake pans and line bottoms with parchment paper.
  • Whisk together cocoa powder and hot coffee in a medium sized bowl and set aside.
  • In a large mixing bowl, combine flour, sugar, baking soda, baking powder and salt. Add eggs, buttermilk, melted butter and vanilla extract and beat on medium speed for 3 minutes. Scrape sides of bowl as needed. Stir in coffee and cocoa mixture. The cake batter will be very runny.
  • Divide batter between the two cake pans and bake for 32-35 minutes, until cake is springy when touched and no dent is left behind. A wooden toothpick tester must come out clean.
  • Cool cakes completely in pans on wire racks. When cakes are no longer warm, cover the pans tightly with plastic wrap or aluminum foil and chill for at least 2 hours.

Chocolate Ganache Frosting

  • *Make Ganache Frosting ahead of time as it needs about 2 hours to cool.*
  • In a medium-sized saucepan, heat and stir the cream, butter, sugar and salt over low heat until mixture nearly comes to a boil. Do not allow it to boil. Remove from heat and stir in chocolate until completely smooth. Stir in vanilla.
  • Allow mixture to cool to room temperature (about 2 hours), until it reaches a spreadable consistency, stirring occasionally.

Vanilla Rainbow Frosting

  • Beat the butter and salt in a mixing bowl on medium speed for 3-5 minutes until the butter appears nearly white. Add the sifted powdered sugar and vanilla and mix on low speed until blended. Scrape bottom and sides of mixing bowl with a rubber spatula and mix again briefly. Add the heavy cream and mix on medium speed until the frosting is creamy.
  • Divide the frosting into five bowls (about 130 g each) and color each frosting a different color of your choice. Begin coloring with minimal amounts of food coloring. Transfer frosting to five plastic food storage bags.

Cake Assembly

  • Turn chilled cakes out of pans and remove parchment paper. Slice each cake horizontally into three layers. Ideally, you would use a cake leveler to cut the layers but a long, serrated bread knife works as well. Don’t worry if the layers aren’t perfect, the cake will still be beautiful.
  • Place the bottom layer on a cake platter and lay strips of parchment paper under the edges to make cleaning the platter easier afterwards.
  • Cut a small corner off your first bag of frosting. Pipe the frosting onto the cake in large swirls and smooth it out with an offset spatula or a butter knife. Repeat process with remaining cake layers and frosting.
  • When the chocolate ganache frosting has thickened into a pudding-like spreadable consistency, you can use it to frost the cake. If it is still too soft, you can set it in the freezer for a few minutes. If it is too hard, you can heat it in the microwave for 5 seconds.
  • Start by frosting the sides of the cake. Holding your palette knife parallel to the cake, apply plenty of frosting to the sides, filling in any holes and uneven spots with the frosting. Smooth out the frosting and remove excess by scraping the sides of the cake with palette knife or cake scraper (or a ruler). Repeat until the sides are nicely covered. Then frost the top of the cake and scrape smooth.
  • Decorate cake with colored sprinkles as desired. Chill cake over night before cutting into slices and serving.

Notes

Special equipment: 2 round cake pans 8 inches (20 cm) in diameter, cake leveler, food coloring (preferably gel or paste), colored sprinkles, palette knife (straight or offset spatula) or cake scraper (or a ruler), Lazy Susan (turntable).