Preheat the oven to 350°F. Butter two 8-inch round baking pans, then place a round of parchment, cut to fit neatly inside, on the bottom of each pan, and butter the parchment. Add a couple of tablespoons of flour to one of the pans and tilt the pan so that the inside gets completely lightly coated with flour. Turn the remaining flour into the second pan and repeat. Turn the pans over and gently tap out all excess flour.
In a small bowl, whisk together the cocoa powder and coffee. Set aside.
In a medium bowl, combine the 2 cups flour, baking soda, baking powder, and salt.
In a large bowl, place the butter and sugar. Beat with an electric mixer until light and fluffy, about 3 minutes. Add the eggs one at a time. Beat in the vanilla. Scape down the sides and add the coffee mixture. Beat until incorporated.
Add half of the dry ingredients, and blend until almost incorporated, then add the sour cream and beat until almost incorporated, then add the remaining dry ingredients, and continue beating until everything is well blended. Scrape down the sides and beat for a few more seconds.
Divide the mixture evenly between the two prepared pans and bake for 45 to 55 minutes (if you use a 9-inch cake pan, start checking at 40 minutes), until a wooden skewer inserted in the middle of the pan comes out pretty clean (no undercooked batter attached!)
Cool the cakes in the pans on a wire rack for 10 to 15 minutes, then carefully turn them out and cool completely on wire racks.
To assemble the cake, place one cake layer upside down on a round of cardboard or a flat plate. Using an offset spatula or a butter knife frost the top of the with a generous layer ofthe Chocolate Buttercream. Place the second cake layer upside down on top of the bottom layer, and frost the top and the sides of the cake of the with a thin layer of the Chocolate Buttercream. Place the cake in the freezer for 15 minutes, or in the refrigerator for 30 minutes to firm up the thin layer of frosting.
Remove the cake from the fridge and frost the top and the sides with a generous layer of buttercream, making it as smooth or as swoopy as you like.
If you would like to add the topping, make sure the top is very smooth. Warm the sauce until it is pourable, but still thick and pour it slowly in a circle over the top working your way towards the edges. Use a spoon, knife or offset spatula to help the topping drip over the sides in random spots, to create gorgeous chocolatey drips. Place the cake in the fridge for at least 15 minutes to firm up the sauce.
Cut into slice, serve, and modestly accept all of the compliments that will be coming your way.