Preheat oven to 350º Fahrenheit. Grease a 9×13 baking pan with cooking spray. Line with parchment paper if you’d like.
For the cake, put flour, sugar, cocoa, baking powder, baking soda and salt in a bowl and mix well.
In a stand mixer, mix the buttermilk, oil, eggs, vanilla together. Slowly add in the mix of dry ingredients while the mixer is on low. Add the coffee and make sure everything is combined. The batter will be very thin.
Pour the batter into the prepared pans and bake for about 30-35 minutes or until a toothpick inserted in the middle comes out clean.
Allow the cake to cool on a wire rack.
In a stand mixer using the whisk attachment, cream the butter for 2-3 minutes until it is almost white in appearance.
Add the powdered sugar and cocoa powder, 1 cup at a time, while mixing on low speed. Cover the mixer with a dish towel, if needed, to prevent powdered sugar from blowing out of the bowl.
Add in the salt and vanilla. Continue mixing on low and add the heavy cream, one tablespoon at a time.
Turn the mixer to medium and beat for 1-2 minutes, until the buttercream is light and fluffy.
Frost the cooled cake.
Store in an airtight container at room temperature.