Preheat your oven to 350°F (175°C). Ensure the oven rack is in the middle position.

Lightly grease three 8-inch cake pans with baking spray or unsalted butter. Line the bottom of each cake pan with a parchment paper round.

In a large mixing bowl, cream together the butter and sugar using an electric mixer until the mixture is light and fluffy.

Beat in the egg yolks one at a time, ensuring each is well incorporated before adding the next.

In a separate medium-sized bowl, whisk together the flour and baking powder.

Slowly add the dry ingredients to the butter mixture, alternating with the milk and vanilla, beginning and ending with the dry ingredients. Mix until just combined.

In another clean bowl, using clean beaters, whip the egg whites and salt at high speed until stiff peaks form.

Gently fold the whipped egg whites into the cake batter until well combined.

Split the batter into two equal portions. Mix the cocoa powder into one half, stirring until well combined.

Spoon both vanilla and chocolate batters alternately into the prepared cake pans. Using a butter knife, gently swirl the two batters together to create a marbled effect. Be careful not to touch the bottom of the pan.

Place the cake pans in the oven and bake for approximately 30 minutes or until a toothpick inserted into the center of the cakes comes out clean.

Remove from the oven and allow the cakes to cool in the pans for 10 minutes. Then transfer them to a wire rack, remove from pan, remove the parchment paper round, and cool completely.

Prepare the Frosting: In a large mixing bowl, beat the butter until smooth and creamy. Gradually add the powdered sugar and mix until smooth. Add the heavy cream, vanilla, and salt, mixing well. Stir in the cocoa powder until fully combined. In a microwave-safe bowl, melt the chocolate wafers in 30-second intervals, stirring between each until smooth. With the mixer on low, slowly pour in the melted chocolate, then increase the speed and mix until smooth and well combined.