Preheat the oven to 350 F. Butter two 8 x 2 round cake pans. Line with parchment paper, and then butter and flour the pans.

In the bowl of a stand mixer with a paddle attachment, sift flour, sugar, cocoa, baking soda, baking powder, and salt on low speed until combined. In a separate bowl, combine the buttermilk, oil, eggs, and vanilla.

Add wet ingredients to the dry ingredients in the mixer and stir on low until combined. Add the coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula.

Pour the batter into the prepared pans and bake for about 35 or until a toothpick inserted into the center comes out clean. Let cool for 30 minutes before removing from pans to cool completely on wire racks.

While cake is cooling, make the frosting. Chop the chocolate and heat over double broiler just until melted and set aside until cooled to room temperature.

Beat the butter on medium-high speed in a stand mixer until light and fluffy, or about 3 minutes. Add the egg yolk and vanilla and continue beating for an additional 3 minutes. Turn the mixer on low, and gradually add the confectioners sugar.

Increase speed to medium and beat until smooth and creamy, scraping down the bowl as necessary. Dissolve the instant coffee in 2 teaspoons of the hottest tap water.

Reduce speed to medium and add the chocolate and coffee. Mix until blended. Use immediately to frost the cooled cake.