Mix all the dry ingredients in a large bowl ( sugar, flour, cocoa powder, baking soda and salt). Stir with a whisk until everything is well incorporated.
Add the eggs, buttermilk and vanilla and slowly mix everything until just combined (it's ok if it's lumpy at this point).
Add the coffee and stir with a whisk until just combined. Again, it's ok if the batter is slightly lumpy at this point, as long as you don't have big clumps of trapped flour. Some people like to sift the batter into a new bowl at this point, but honey... you do you ;)
Pour batter into liberally buttered cake pans, and bake for 20-40 minutes, depending on the size of your pans. Shallower cake pans will be closer to 20 minutes while deeper pans can take up to 40 minutes.
Frosting
Cream the butter in a stand mixer. Add cocoa powder and mix well, scraping down the bottom of the bowl.
Add the icing sugar, salt and vanilla and beat together for about 2-4 minutes. Scrape the sides and bottom of the bowl a couple of times.
Add icing only once the cakes have cooled down completely.
Notes
I'll cover the black velvet icing, which covers the cake in the photographs, in another recipe on my blog.