For the cake:
In a large, heat proof mixing bowl, whisk together granulated sugar and brown sugar (lumps are fine), then whisk in cocoa powder, baking soda and salt. Carefully pour boiling water in to cocoa mixture and immediately whisk to blend well, about 15 - 20 seconds. Allow to cool 5 minutes.
Using an electric hand mixer set on low speed blend in canola oil until combined, about 10 seconds. Add eggs to cocoa mixture and blend on low speed just until combined, about 5 seconds. Add egg yolks and vanilla extract and blend just to combined. Then add flour and blend only 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 (after about 20 minutes of cooling,
Trim tops from cake to form an even layer (they are quite domed so you don't need to make it completely level across - just add a little more frosting to the edges that aren't as level so you don't have to cut off so much cake). Add frosting to top of one round then top with second cake and frost top and sides of cake. Store in an airtight container.
Chocolate Buttercream Frosting