• Preheat the oven to 350F. Butter and flour 2 9-inch cake pans. Place a piece of parchment paper that’s been trimmed to fit in the bottom of each buttered and floured pan. Lightly spray the parchment paper with nonstick cooking spray.
• Whisk together the flour, cornstarch, baking powder, and salt in a medium bowl.
  1 3/4 cups all-purpose flour, 1/4 cup cornstarch, 2 teaspoons baking powder, 1/2 teaspoon salt
• Use a handheld electric mixer to cream together the butter and sugar in a large bowl. Beat in the eggs and vanilla until light and fluffy.
  1 cup unsalted butter, 1 1/3 cups granulated sugar, 3 large eggs, 1 Tablespoon pure vanilla extract
• Alternate between adding the dry ingredients and the milk until they’re both fully added, being careful not to over-mix. The batter will be thick.
  2/3 cup whole milk
• Remove 1 cup of batter to a bowl, and stir in the melted chocolate.
  4 ounces semi-sweet chocolate
• Divide the yellow batter evenly between the 2 prepared cake pans. Dollop the chocolate batter on top, and then gently swirl it in with a spatula.
• Bake until the cakes are golden along the outside and a toothpick inserted in the center comes out clean, about 27 to 34 minutes.
• Cool the cakes for 15 minutes in the pans, and then turn them out onto a wire rack to finish cooling, about 15 minutes. Peel off the parchment paper.

Making the Frosting

• In a small saucepan or skillet, melt the butter over low heat.
  16 Tablespoons butter
• Mix in cocoa powder, stirring until the mixture is nice and thick.
  2 cups unsweetened cocoa powder
• Place chocolate mixture in the bowl of a stand mixer (or a large mixing bowl if using a hand mixer). Add ½ of the powdered sugar, mix until combined.
• Add half of the heavy cream and the vanilla, mix until combined. Continue with remaining powdered sugar and cream.
  4 teaspoons pure vanilla extract
• Mix on medium speed for 5 minutes, until frosting is smooth and creamy.
• Once the cakes are cooled, assemble the layer cake. Place 1 cake round onto a cake platter. Spread a layer of frosting on top. Place the second cake round on top of the frosting. Frost the top and sides of the cake.