Preheat oven to 350 degrees, and grease two 12-cup muffin trays, for a total of 24 cupcakes, or two 8-inch round cake pans. Line the cake pans with parchment paper, then butter and flour the pans.

Sift together the dry ingredients: flour, sugar, cocoa, baking soda, baking powder, and salt, into the bowl of a stand mixer, fitted with the paddle attachment. Stir together until well blended, about 1 minute, on LOW speed.

In another bowl, combine the buttermilk, oil, eggs, and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry ingredients. Continue to run at low speed until well combined, scraping the bottom of the bowl as needed.

Continuing on low speed, slowly pour in the hot coffee, and mix until smooth in consistency. The batter will be VERY runny, compared to most cake batters. Pour the mixture into a glass measuring cup to more easily pour into the muffin tins, filling about ¾ of the way up. This should fit exactly into 24 cups.

Bake at 350 for 25 minutes for cupcakes (35-40 minutes for cake), then allow to cool in the pans for 30 minutes. This will ensure even settling and a better release from the pans. Continue to cool on a baking rack as needed. Set aside until ready to frost.

FROSTING: Chop the chocolate and place in a glass bowl over a pot of simmering water, stirring slowly until just barely melted. Remove from the heat, stirring until all pieces are smoothly combined, then set aside to cool.

In the bowl of an electric mixer fitted with the whisk attachment, beat the butter on medium-high speed until yellow and fluffy, about 3 minutes. Add the egg yolk and vanilla, continuing to beat for another 3 minutes. Reduce the mixer to low speed, then slowly add the confectioner's sugar and continue to mix for another minute or so until combined, scraping down the bowl as needed. Last, add 2 tsp of the hottest tap water to the instant espresso powder, and slowly add that and the melted chocolate to the frosting mixture, until smooth in consistency.

Frost the cupcakes! Use a large zip-top bag, snipping a small opening in one corner to pipe the frosting on. You can also just spread it - the frosting is so delicious it doesn't matter what it looks like!

Frost the Cake! Place one layer, flat side up, on a flat plate or cake pedestal. With a knife or offset spatula, spread a thin layer of buttercream on the top only. Place the second layer on top, flat side up, and spread the frosting evenly first on the sides and then on the top of the cake. Cut in wedges and serve at room temperature.