In the bowl of a stand mixer, cream together the sugar, salt, butter, vanilla and egg.
In a separate bowl, sift together the flour, baking soda and cocoa. Add this to the first mixture alternately with the coffee and buttermilk.
Drop the batter by teaspoonfuls onto a non-stick, greased cookie sheet. You can also line the sheet with parchment paper or a silicone baking mat. Bake the cookies at 400 degrees for 7-8 minutes. Set the cakes aside to cool.
In a stand mixer bowl, whip the softened butter on medium speed until it is almost white in appearance.
Add the powdered sugar and cocoa powder. Mix on low speed.
Add in the hot coffee and whip the buttercream on medium speed for 3-4 minutes, or until light and fluffy.
Choose two whoopie pies that are about the same size. Spread the buttercream between the two cakes.
This should make about 18 whoopie pies that are 2 1/2" in diameter.
Store in an airtight container.
*If the whoopie pie batter seems to thin, it may be because of the fat content of your milk. I used 1% milk, and didn't need to add any flour. You can add a bit of flour to thicken the batter, if needed, a teaspoon full at a time.*To make the strongly brewed coffee, I used 3 tablespoons of ground espresso and brewed it with 3/4 cup water. You can also use espresso powder, which is instant espresso that easily dissolves in hot water.