Preheat oven to 350°F. Butter two 9-inch round cake pans and line the bottoms with parchment paper. Butter the parchment papers.

In a large heat-proof bowl, whisk together granulated sugar, brown sugar, cocoa powder, baking soda, and salt. Carefully pour in boiling water and whisk until smooth. Let it cool for 5 minutes.

Using an electric mixer on low, blend in vegetable oil and melted butter. Add eggs, egg yolks, and vanilla, mixing until combined. Add flour, then milk and sour cream, mixing gently.

Divide batter evenly into prepared pans. Bake for 29-34 minutes or until a toothpick inserted into the center comes out clean. Cool in pans for 5 minutes then invert onto wire racks to cool completely.

Trim cake layers if needed. Frost with chocolate buttercream and assemble. Store the cake in an airtight container.