Preheat the oven to 350ºF (180ºC). Line the bottoms of two 6-inch cake pans with parchment paper. Grease the insides and dust with a mixture of equal parts flour and cocoa powder. Wrap the cake pans with damp cake strips if you have them.
Sift the flour, cocoa powder, baking powder, baking soda and salt together in a medium-sized bowl. Add the sugar and stir until well blended.
Combine the the egg, egg yolk, buttermilk, vegetable oil and vanilla extract in a small bowl. Whisk lightly to combine.
Add the egg mixture to the dry ingredients and beat, on low speed with an electric mixer, until combined. Stop the mixer. Scrape the sides and the bottom of the bowl with a spatula. Turn the mixer up to medium speed and beat for 30 seconds. Add the hot coffee and stir until well combined.
Pour the batter into the prepared cake pans, dividing batter evenly between the pans. Bake until a toothpick inserted into the center of the cakes comes out clean, about 25-28 minutes without cake strips or about 28-32 minutes with cake strips.
Let the cakes cool in the pans for 10 minutes then remove the cakes from the pans and transfer to a cooling rack.
Trim the cake layers to your desired thickness, from 1-inch to 1 1/2-inches thick. Fill and frost with chocolate buttercream frosting.