Sift the flour, cocoa powder, salt, baking soda and baking powder into a large bow then whisk together and set aside.
Add the egg, buttermilk, and coffee to a medium bowl and whisk together.
Cream the room temperature butter then add the sugar and mix until light ad fluffy.
Add the dry and wet mixtures in three alternating batches to the butter starting with the dry while mixing on low. Mix until just combined then use a spatula to scrape the bowl down and do any final mixing.
Use a heaping tablespoon or small ice cream scoop to transfer dollops of batter onto a baking sheet lined with parchment paper. Wet your fingers with water and tamp down any bits of batter sticking out.
Bake for about nine minutes at 375F or until the tops spring back when gently pressed.
For the Filling
Add egg whites, sugar and salt in a bowl.
Give the mixture a brief whisk.
Place the bowl over a pan of simmering water. Make sure the water does not touch the bowl.
Whisk the egg whites occasionally while it warms up. When the mixture has warmed, whisk it constantly. You will heat the mixture until it reaches an internal temperature of 160ºF or until it's not grainy between your fingers.
Transfer the bowl to a stand mixer, mix with a whisk attachment until you get room temperature glossy peaks.
Switch to a paddle attachment and while the mixture is running on low, add tablespoon sized dollops of room temperature butter making sure to let the butter incorporate before you add the next piece.
Once all the butter is incorporated, go ahead and add the vanilla extract. You can transfer it to a piping bag or to a ziplock bag to store in the refrigerator or freezer.
For the Assembly
Pipe the buttercream onto the back of a cake then sandwich together with another cake round.
To reactivate the buttercream after it has been frozen, allow it to come to room temperature then whip it.If you start adding butter and the mixture becomes soupy don't panic!!! It will be soupy until ALL the butter has been added.Don't get fat in your bowl, make sure it's clean (I actually give mine a quick rub with a lemon then wipe with a paper towel)Don't let any yolks get into your whites when you separate the eggs. The easiest avoid yolks is to separate the eggs one at a time into separate bowls and then pour the white in as you go along.I add flavorings at the end, especially if I'm making a batch of chocolate SMBC, where there's a considerable amount of liquid going in.