Grease two 9-inch round pans with 1/2-inch sides using room-temperature butter
Cut parchment paper to fit pan bottoms; place parchment in prepared pans
In a bowl, whisk cake flour, baking powder, and salt
In a stand mixer, cream butter, granulated sugar, and brown sugar on medium speed until light and fluffy
Add eggs and egg yolks one at a time, mixing well after each addition
Stir in vanilla extract
Divide batter into two bowls
To one bowl, add buttermilk and vanilla extract
To the second bowl, melt chocolate over double boiler; whisk into batter with 3/4 cup flour
Strain cocoa powder into the second bowl, mixing until smooth
Pour vanilla batter into prepared pans followed by chocolate batter
Gently ladle remaining chocolate channels across vanilla layer, using a spatula to create marble swirls
Bake at 350°F (175°C) for 22-27 minutes until a toothpick comes clean
Cool in pans for 10 minutes, then invert onto cooling racks
Sift confectioners’ sugar and cocoa powder into a bowl
Whisk in heavy cream until smooth, then spread between cooled layers