just under 1 cup milk - use enough to fill 1 cup total of the white vinegar and milk
Preheat your oven to 350° F.
Coat a 9x13 inch pan with butter. If using two cake pans, coat with butter, then press waxed paper on bottom of pans. You can use your fingers and a little pat of butter, but I find it easiest to just grab the block of butter and rub it all over the bottom and sides of the pan.
In a large sized bowl, cream the butter and sugar together with a mixer, until combined. Slowly and carefully, mix in the cocoa powder. Add the vanilla and eggs to the mixture and continue mixing until combined.
In a separate medium sized bowl, combine flour, baking soda, and salt. Slowly mix in some of the flour to the wet mixture. Next, slowly mix in some of the sour milk. Continue alternating, until all ingredients are combined, ending with flour.
Stir in the hot coffee or boiling water. Pour into the pans and bake for 35-40 minutes or until a toothpick comes out clean. Remove from oven and let cool for ten minutes.
In a large bowl, combine all ingredients. Slowly and carefully stir together until combined. Spread over the cake, once it has cooled (if time allows, freeze cake before frosting. Let come to room temperature before serving)