Preheat oven to 350 degrees F. Liberally grease two 9" round baking pans with cooking spray; set aside. I like to also add in a round piece of parchment paper.

In the bowl of a stand mixer, whisk together the sugar, flour, cocoa powder, baking powder, baking soda, and salt until combined. Add in the oil, vanilla, buttermilk and eggs and beat on low speed until combined. Gently and slowly pour in the hot coffee until the batter comes together. Batter will be quite thin; this is okay.

Pour the batter evenly into the pans. Bake the cakes for 25-30 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs. Cool completely.

Make the frosting while the cake cools: In a large bowl, combine the cubed butter and chopped chocolate bar. Microwave on HIGH power for about 30 seconds, stirring until completely melted and smooth. Cool for 5 minutes. Add the powdered sugar, milk and vanilla to the chocolate mixture and beat it on HIGH speed for 5-7 minutes or until the frosting is light and fluffy. It's easiest to do this in a stand mixer. Frosting may be a little runny; this is normal. Refrigerate frosting for 15-20 minutes or until thickened.

Place a cake layer top-down on a cake stand or plate. Spread with 1/3 of the fudge frosting out to the edges. Top with remaining cake layer and frost the remainder of the cake. Top with sprinkles if desired.