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

In a large bowl, whisk together sugar, brown sugar, cocoa powder, baking soda, and salt. Carefully add boiling water and whisk until well blended. Let cool for 5 minutes.

Using an electric mixer on low, blend in oil and melted butter. Add eggs, yolks, and vanilla, and mix until combined. Mix in flour, then add milk and sour cream, mixing just until smooth.

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

Trim cakes if necessary. Spread frosting on top of one cake layer, then place the second layer on top. Frost the top and sides of the cake. Store in an airtight container.