Preheat oven to 350 degrees F.
Grease 2 (8x2-inch) or 3 (6x2-inch) round cake pans with butter and flour or cooking spray and lining the cake pans with parchment paper.
Combine the sugar, flour, cocoa powder, baking soda, baking powder, and salt in a large bowl or stand mixer fitted with a whisk attachment.
Beat in eggs, buttermilk, coffee, oil, and vanilla and beat on medium speed for 2 minutes, the batter is going to be very thin.
Pour 2 cups of batter into each 6-inch pan or divide evenly between the two 8-inch pans.
Bake 6-inch cakes for 30-35 minutes and 8-inch cakes for 40-45 minutes until toothpick comes clean from center.
Cool in pans for at least 10 minutes and loosen from sides of pan before transferring to a cooling rack to cool completely.
Whip frosting ingredients together in a large bowl or stand mixer fitted with a whisk attachment on medium-high until light and fluffy, about 4 to 5 minutes.
Add about 1½ to 2 cups of frosting to the top of one of the layers of cake, spreading evenly over it, then topping with the second layer of cake. Use a frosting spatula to generously frost the outside of the cake. Serve within 2 hours or store in the fridge for up to 24. Top with candy just before serving.