In beater bowl, cream together butter, sugar and cocoa. Add egg and blend.
Stir in sour milk and hot water to which 1 teaspoons baking soda has been added.
In another bowl, mix together the baking powder, salt and flour. Add all at once to the creamed mixture. Add vanilla. Stir to incorporate all the flour mixture, then beat with mixer at high speed for 3 minutes, scraping down sides of bowl with spatula.
Spoon into paper-lined muffin tins filling 2/3 full. Bake at 350° for 18-20 minutes. Frost with Fudge Icing.
1/2 cup butter, 1 egg, 1/4 cup cocoa, 2 cups icing sugar, milk (see below).
In bowl, cream together butter and cocoa. Add egg and blen well. Mix in icing sugar, then using beater on high, combine throughly. If too stiff, add 1 tsp milk at a time until desired consistency is reached. Fudge icing should be thick and moist rather than light and fluffy.