Bake the Cake: Preheat oven to 325 degrees F. You will need two 8” round cake pans, line w/ parchment paper at the bottom, greased and coated w/ very fine bread crumbs.
Into a large mixing bowl, sift together flour, sugar, cocoa, baking powder, baking soda and salt.
Make a wide well in the middle of the dry ingredients. Add in the following order, without mixing, the buttermilk, oil, eggs and vanilla. With a large wire whisk, beat until smooth. Mix in coffee. Set aside.
Gently pour the batter into the pans. Bake for 30 minutes at 325 degrees F, or until the top springs back when lightly pressed with a fingertip. Cool completely.
For the frosting: Chop the chocolate and melt with a double boiler. Cool to room temperature.
Beat the butter until light yellow and fluffy. Add egg yolk and vanilla. Gradually add the sugar. Dissolve the coffee in 2 t hot water. Add the chocolate and coffee to the butter mixture until blended. Do not whip! Spread immediately on cooled cake. Top with candied orange.