1. Preheat the oven to 350 degrees F. Grease 2 (8-inch) round cake pans. Line with parchment paper, then butter/spray with cooking spray.
2. In a large mixing bowl, beat together the eggs, buttermilk, greek yogurt, canola oil, and vanilla until smooth. Add the flour, sugar, unsweetened cocoa powder, baking soda, baking powder, and salt and mix until there are no longer any clumps of flour. Add the hot coffee and mix until combined. The batter should be pourable, but not super thin. Stir in the chocolate chips, if using.
3. Pour the batter among the 2 cake pans and bake 20 to 25 minutes, until the tops are just set and no longer wiggly in the center. Remove and let cool five minutes, then run a knife around the edges of the pan. Invert the cakes onto a cooling rack. Cover and let the cakes cool completely before frosting.
Whipped Chocolate Buttercream
1. In a large mixing bowl, beat together the butter and powdered sugar until the butter is light and fluffy, about 4 minutes. Add the cocoa powder and vanilla and beat, scraping down the sides as needed, another 2 minutes or so, until there are no streaks of white. Add 3 tablespoons of the heavy cream and whip the frosting for 2-4 minutes or until light and fluffy. If desired add the remaining tablespoon of the heavy cream (I normally do) and whip until combined. Taste the frosting and add more powdered sugar if you like a sweeter or thicker frosting.
2. Place 1 layer, flat side up, on a plate or cake stand. With a knife or offset spatula, spread the top with frosting. Place the second layer on top, rounded side up, and spread the frosting evenly on the top and sides of the cake. Decorate with sprinkles, chocolate curls, or whatever your heart desires.
*To quickly bring eggs to room temperature place them in a bowl and fill with warm water. Let them sit five minutes.