Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing with butter and lining with parchment paper.

In a large mixing bowl, whisk together granulated sugar, brown sugar, cocoa powder, baking soda, and salt until there are no lumps.

Carefully stir in boiling water until smooth; let the batter cool for about 5 minutes.

Mix in vegetable oil and melted butter, then add eggs, yolks, and vanilla extract, stirring gently.

Gradually fold in flour, then add milk and sour cream until the batter is smooth and lump-free.

Divide the batter between the prepared pans and bake for 29-34 minutes, or until a toothpick comes out clean or with a few moist crumbs.

Allow the cakes to cool in the pans for 5 minutes, then invert onto wire racks to cool completely.

Level the tops if necessary, then frost with Chocolate Buttercream Frosting between layers and around the entire cake.