Preheat the oven to 350 degrees. Butter two 8-inch round cake
pans (for cupcakes, see note). Line with parchment paper, then
butter and flour the pans.
Sift the flour, sugar, cocoa, baking soda, baking powder, and salt
into the bowl of an electric mixer fitted with the paddle attachment
and mix on low speed until combined. In another bowl, combine
the buttermilk, oil, eggs, and vanilla. With the mixer on low speed,
slowly add the wet ingredients to the dry ones. With mixer still on
low, add the coffee and stir just to combine, scraping the bottom
of the bowl with a rubber spatula. Pour the batter evenly into the
prepared pans and bake for 35 to 40 minutes, until a cake tester
comes out clean. Cool in the pans for 30 minutes, then turn them
out onto a cooling rack and cool completely. (Not to worry; the top
will sink a little in the center.)
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.
Note- For cupcakes, reduce bake time to 25-30 minutes.