Heat oven to 350°F. Grease and flour two 9-inch round pans, one 13x9x2-inch baking pan or line a muffin pan with 24 cupcake liners.
Stir together sugar, flour, cocoa, baking powder, baking soda and salt in large bowl.
Add eggs, buttermilk, oil and vanilla; beat on medium speed of electric mixer 2 minutes. Stir in boiling coffee (batter will be very thin). Mix until well combined.
Pour into prepared pans.
Bake 30 to 35 minutes for round pans, 35 to 40 minutes for rectangular pan, approximately 20-25 minutes for cupcakes, or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pans to wire racks. Cool completely prior to frosting. (Cake may be left in rectangular pan, if desired.)
Beat butter in medium bowl. Add powdered sugar and cocoa alternately with milk and coffee, beating to spreading consistency (additional milk may be needed depending up your desired consistency). Stir in vanilla and salt (if using).