Cream together butter and sugar. Add egg yolks and mix well. Pour hot water over chocolate mix and stir until dissolved. Pour into butter mixture along with milk.
Sift flour, baking powder and salt. Stir into butter mixture. Add vanilla and stir.
Whip egg whites and fold into batter.
Spoon batter into greased muffin tins filling each cup at least 3/4 full. I filled them full until the batter came just to the top of the cup.
Bake at 375 degrees for 20 minutes or until inserted toothpick comes out clean. Cool slightly and remove from tins.