Preheat oven to 350°F. Grease two 9-inch round cake pans and line with parchment paper. Set aside.

In a large bowl, whisk together sugar, brown sugar, cocoa powder, baking soda, and salt. Carefully add boiling water and whisk until smooth. Let 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. Gradually add flour, then stir in sour cream and milk until smooth.

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

Frost with buttercream frosting, assemble, and serve.