Preheat oven to 350 degrees F. Liberally grease a 9x13" rectangular baking pan with cooking spray; set aside. Or, if using another pan, spray liberally with cooking spray (such as two 9" rounds, or a Bundt pan, or line 2 muffin tins with 24 paper liners).
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 pan or pan(s) or cupcake liners. Bake the 9x13" rectangular pan for 30-35 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs. Cool completely. (Other pan times will be at the end of this recipe).
To make the frosting: 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.
Spread the frosting onto the cooled cake and smooth out the top. Sprinkle liberally with sprinkles, if using. Cover and refrigerate for 1-2 hours to set before cutting into slices.
FOR CUPCAKES: Bake for 15-20 minutes.
FOR BUNDT CAKE: Bake for 55-65 minutes.
FOR TWO 9" ROUND CAKES: Bake for 25-30 minutes.