Preheat the oven to 350° F. Spray 3 8 inch round baking pans with flour-added cooking spray or just grease and flour. Alternatively, you could use 2 slightly deeper 9 inch pans, but the cakes will be thicker and need a little more time in the oven.
Have ready two large mixing bowls. In the first bowl, mix together all dry ingredients for the yellow cake batter (flour through salt). In the second bowl, mix together all dry ingredients for the chocolate batter (flour through salt).
Make the yellow batter first. Whisk the eggs, yolk, vanilla and 2 tablespoons of the milk together in a small bowl. Add the butter to the dry ingredients and mash it around, then stir in the egg mixture until it’s as blended as you can get it. Add the remaining buttermilk and beat with a handheld mixer until smooth.
Repeat the same procedure with the chocolate batter. Whisk the egg, vanilla and all the buttermilk together in a small bowl. Add the butter to the dry mixture and mash it around. Add the egg mixture and blend it as much as you can, then add the coffee and beat with a handheld mixer until smooth. Now you should have a bowl of vanilla batter and a bowl of chocolate batter.
Starting with the vanilla (it’s thicker), layer the two flavors of batter into the pans, dividing evenly among the pans.
Bake the cakes for about 25 minutes or until a toothpick inserted in the center comes out clean. Let cool in the pans for 10 minutes, then carefully invert onto a cooling rack. Let cool completely, then fill with Mocha Filling, stack and frost with Chocolate Frosting.
Make the mocha filling first. In a large mixing bowl, mix the butter, coffee, confectioners’ sugar and about 1 tablespoon of the cream. Beat until well mixed, then gradually add the cream, scraping sides of bowl often, until you get a smooth, spreadable consistency. Frost the tops of two cakes.
Make the chocolate frosting. In a large mixing bowl, mix the butter, sugar, cocoa, salt and 3 tablespoons of the cream as well as you can, then beat with a hand-held mixer until pasty and thick. Continue adding remaining cream until you get a nice, smooth, spreading consistency