The Song of the Holly Fairy
Preheat the oven to 350 degrees F. Butter two 8-inch x 2-inch round cake pans or a 9 x 13 inch baking dish. Spray with cooking spray. Sift the flour, sugar, cocoa, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with a paddle attachment and mix on low speed until combined. In another bowl, combine the buttermilk, oil, eggs, and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry. With mixer still on low, 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 or baking dish and bake for 35 to 40 minutes, until a cake tester comes out clean. Cool in the pans or baking dish for 30 minutes. If using the cake pans, then turn them out onto a cooling rack and cool completely.
Chop the chocolate and place it in a heat-proof bowl set over a pan of simmering water. Stir until just melted and set aside until cooled to room temperature.
In the bowl of an electric mixer fitted with a paddle attachment, beat the butter on medium-high speed until light yellow and fluffy, about 3 minutes. Add the egg yolk and vanilla and continue beating for 3 minutes. Turn the mixer to low, gradually add the confectioners' sugar, then beat at medium speed, scraping down the bowl as necessary, until smooth and creamy. On low speed, add the chocolate and coffee to the butter mixture and mix until blended. Don't whip! Spread immediately on the cooled cake.