Preheat the oven to 350 degrees and butter two 8-inch round cake pans. Grab some parchment paper and trace two outlines of the cake pan. Cut out the outlines (Perfect circles, voilá!) and place a sheet of parchment into each of your pans. Then butter and flour the pans again with the parchment paper in the bottom.
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 a separate bowl or large measuring cup, combine the buttermilk, oil, eggs, and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry ingredients.
Now grab your brewed coffee, and with the mixer still on low, slowly add the coffee. Mix just to combine, scraping the bottom of the bowl with a spatula to make sure everything's thoroughly mixed. The resulting batter will be quite thin.
Divide the batter evenly between the two pans and bake for 35 to 40 minutes, until a toothpick comes out clean. Let the cakes cool in the pans for 30 minutes, and then (carefully) turn them out onto a cooling rack to cool completely. If you didn’t use the parchment paper, definitely grab a butter knife to loosen the cakes first. The moist crumb makes these guys pretty delicate.
While the cakes are done cooling, prepare your chocolate frosting.
Measure out your chocolate (if using bars, chop into pieces first) and place in a heat-proof bowl set over a small pot of simmering water. Stir until just melted and set aside until cooled to room temperature. Another cheat—it can be a little warm. Your frosting will be safe.
While that's happening, use your electric mixer + paddle attachment to beat 2 sticks of 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.
With the mixer on low, gradually add the confectioners' sugar. The original recipe says to sift it—but really, just beat the frosting a little longer to get rid of any clumps! Continue to beat at medium speed, scraping down the bowl as necessary, until the frosting is smooth and creamy. Then add in the chocolate and beat until combined. Be careful not to over-whip the frosting.
At this point, you can dissolve the instant coffee powder in 2 teaspoons of hot water, and add it into the frosting. This step is optional.
Now, to frost the cake! Place the first cake layer on a flat plate or cake pedestal. Ina says flat side up. I say whichever the less mauled side is should be facing up. With a butter knife or offset spatula, spread with frosting—a little more than a ¼-inch thick. Carefully place the second layer on top, and spread the frosting evenly on the top and sides of the cake.