1. Preheat the oven to 350°F. Spray two 6-inch x 2-inch round non-stick cake pans with non-stick baking spray.
2. In the KitchenAid bowl, add the flour, sugar, cocoa, baking soda, baking powder, and salt. Using paddle attachment, slowly mix on low speed until combined.
3. In a separate bowl, combine the buttermilk, vegetable oil, eggs, and vanilla.
4. With the mixer on low speed, slowly add wet ingredients to the KitchenAid bowl of dry ingredients.
5. Slowly add the cup of espresso and stir to combine. Make sure to scrape the bottom of the bowl with a rubber spatula.
6. Pour batter into prepared baking pans and bake for 35 to 40 minutes, or until wooden toothpick inserted in center comes out clean.
7. Cool cakes on cooling racks.
8. Repeat steps if making a multiple layered cake.
9. Frost with chocolate buttercream.

For the Chocolate Buttercream

1. Break chocolate into pieces and melt over a double-boiler until just melted. Set aside until cooled to room temperature.
2. In the KitchenAid mixer fitted with paddle attachment, beat butter on medium-high spread until fluffy. About 2 to 3 minutes.
3. Turn speed down to low, and slowly add confectioners’ sugar to bowl.
4. Add egg yolk, espresso liquid, and vanilla extract, and continue beating for 2 minutes.