Preheat oven to 350°F.

Grease or spray (3) 9” round baking pans. Line the bottom of each pan with parchment paper.

Using a large bowl, add flour, sugar, brown sugar, baking powder, baking soda and salt; whisk until well combined.

Add eggs, egg yolks, vegetable oil, sour cream and vanilla; whisk until well combined.

Divide batter evenly into 3 mixing bowls.

Add ½ cup of boiling water into one of the bowls; whisk together until well combined. Transfer batter into one of the prepared baking pans. This will be the “vanilla” layer.

Using a small bowl, whisk together ¼ cup of cocoa powder with another ½ cup of boiling water. Add this mixture to the batter in one of the two remaining bowls; whisk together until well combined. Transfer batter into one of the prepared baking pans. This will be the “50-50” layer.

Using a small bowl, whisk together remaining ¾ cup of cocoa powder with remaining ½ cup of boiling water. Add this mixture to the batter in the remaining bowl; whisk together until well combined. Transfer batter into one of the prepared baking pans. This will be the “chocolate” layer.

Tap pans lightly on countertop to release any air bubbles.

Bake at 350°F for 20-22 minutes. At this time, check the chocolate layer of cake as it will likely be done. Continue baking the 50-50 layer and vanilla layer for another 5 minutes, or until a toothpick inserted into center of cake comes out clean.

Let cakes cool in pan for 15 minutes and then transfer to a cooling rack until completely cool. (Note: Remove parchment paper from bottom of cakes when transferring them to the cooling rack.)

Using a countertop mixer, beat butter on medium speed until light and fluffy (~3 minutes).

Add powdered sugar 1 cup at a time, mixing on low speed after each addition.

Add salt, vanilla extract and 2 Tbsp of milk; mix on low speed until well combined.

Remove ~½ of the frosting and put it in a separate bowl; set bowl aside.

Working with the remaining frosting, add cocoa powder and remaining 2 Tbsp of milk; stir until well combined. Using a double boiler or microwave, melt chocolate. Pour melted chocolate into the mixing bowl and stir together until well combined. If necessary, feel free to add a bit more powdered sugar or milk here to adjust the consistency of the frosting after adding the melted chocolate.

Using a third bowl, add ~½ of the vanilla frosting and ~½ of the chocolate frosting; stir until well combined. This will be the “50-50” frosting. (Note: There should be more 50-50 frosting than chocolate or vanilla frosting as decorating the sides of the cake will take more frosting.)

Place chocolate layer of cake on large platter. Spread half of chocolate frosting on top.

Place “50-50” layer on top and spread remaining chocolate frosting on top.

Place vanilla layer on top.

Frost the top of the cake with a smooth layer of vanilla frosting.

Using a piping bag fitted with a medium star tip, decorate sides of the cake with the “50-50” frosting.