Heat the oven to 350°F. Grease two 8-inch-by-2-inch round cake pans, line the bottoms with parchment paper, then grease and flour the parchment and sides.

Combine the flour, sugar, cocoa powder, baking soda, baking powder, and kosher salt in a large mixing bowl.

In a separate bowl, whisk together the shaken buttermilk, vegetable oil, room-temperature eggs, and vanilla extract.

With the mixer on low, gradually add the wet ingredients to the dry ingredients. Mix only until the ingredients are combined.

Slowly add the hot coffee and mix until the batter is smooth. The batter will be quite thin, which is normal.

Divide the batter evenly between the prepared pans. Bake for 30–35 minutes, until a cake tester or toothpick inserted into the center comes out clean.

Cool the cakes in their pans for about 30 minutes. Turn them out onto a rack and cool completely before frosting.

Melt the chopped semisweet chocolate gently over simmering water, stirring until smooth. Let it cool to room temperature.

Beat the room-temperature butter on medium-high speed until light. Add the egg yolk and vanilla, then gradually mix in the sifted confectioners’ sugar.

Dissolve the instant coffee granules in the hot water. Add the cooled melted chocolate and coffee mixture to the butter mixture, then beat until smooth and well combined.

Place one cake layer on a serving plate and spread frosting over the top. Add the second layer, then frost the top and sides.