Preheat oven to 350 degrees. Butter two 9-inch round cake pans then line bottom of each with a round of parchment paper. Butter parchment paper. If you’d like more level cakes wrap cake pans with baking strips (which have been soaked and fully saturated with water, see notes below recipe), or you can cut top to level after baking.
In a large heat-proof mixing bowl, whisk together granulated sugar, brown sugar, cocoa powder, baking soda and salt. Carefully pour boiling water in to cocoa mixture and immediately whisk to blend well. Allow to cool 5 minutes.
Using an electric hand mixer set on low speed blend in vegetable oil and melted butter until combined. Add eggs, egg yolks and vanilla extract and blend just to combined. Then add flour and blend until combined, and finish by blending in milk and sour cream just until combined.
Divide mixture evenly among 2 prepared pans. Bake in preheated oven until toothpick inserted into center of cake comes out clean or with a few moist crumbs, about 29 – 34 minutes. Allow to cool in pan 5 minutes, run butter knife around edge of cake and invert onto a wire rack and allow to cool completely.
Trim cake level if needed. Add frosting to the top of one round, then top with the second cake and frost top and sides of the cake. Store in an airtight container.
