Step #1 CAKE: Preheat the oven to 350°F
Butter two 8 inch round cake pans & line them with parchment paper; butter the paper.
Step #2 Dust the pans with flour, tapping out any excess.
Step #3 Mix the flour with the sugar, baking soda, cocoa powder, baking powder & salt at lowish speed.
Step #4 In a separate bowl, whisk the buttermilk with the oil, eggs & vanilla.
Step #5 Slowly beat the buttermilk mixture into the dry ingredients until just incorporated, then slowly beat in the hot coffee until fully incorporated.
Step #6 Pour the batter into the prepared pans.
Step #7 Bake for 35 mins, or until a toothpick inserted in the center of each cake comes out clean.
Step #8 Let the cakes cool in the pans for 30 mins, then invert the cakes onto a rack to cool completely.
Step #9 Peel off the parchment paper.
Step #10 FROSTING: In a microwave-safe bowl, stirring, heat the chocolate at high power in 30 second intervals, until most of the chocolate is melted.
Step #11 Stir until completely melted, then set aside to cool to about room temp.
Step #12 Beat the butter at medium speed until pale & fluffy.
Step #13 Add the egg yolk & vanilla & beat for 1 min.
Step #14 At lowish speed, slowly beat in the confectioners’ sugar, about 1 min.
Step #15 In a small bowl, dissolve the instant coffee in 2 tsps of hot water.
Step #16 Slowly beat the coffee & the cooled chocolate into the butter mixture until just mixd.
Step #17 Evenly spread 1/3 of the frosting over one cake layer to the edge.
Step #18 Top with the second cake layer, rounded side up.
Step #19 Spread the remaining frosting over the top & side of the cake.
Step #20 Refrigerate for at least 1 hr before slicing.