Preheat the oven to 350° F.
Spray one 6-inch and one 9-inch round baking pan (non-stick is preferable) with baking spray.
Line the bottoms of each pan with circles of parchment paper cut to size.
Spray the tops of the parchment with a bit of baking spray.
Sift the flour, cocoa powder, baking soda and baking powder into the bowl of an electric mixer using a sifter or a wire-mesh strainer.
Add salt and sugar to the bowl, and with the mixer set to low speed, beat for 30 seconds or so until combined.
In a separate bowl, whisk eggs, buttermilk, oil, and vanilla.
With the mixer set to low, add the wet mixture to the dry mixture and blend until just combined.
Beat in the coffee until fully incorporated.
Pour the batter into the prepared baking pans (in about a 3:1.3 ratio) and set the cakes in the oven.
Bake cakes for a total of about 30 minutes, until a butter knife inserted into the centres of the cakes comes out clean.
Cool the cakes in their pans on top of wire racks for at least 30 minutes.
Then, invert the cakes on the wire racks, remove the parchment circles, and cool completely.