Preheat oven to 375 degrees F. In top part of double boiler over boiling water heat chocolate, 1/2 cup milk, brown sugar and egg yolk. Stir continuously and remove from heat when smooth and thick. Sift flour, baking soda and salt together into medium bow.
In separate large bowl, beat together butter and sugar, add egg yolks, 3/4 cup milk and vanilla. Beat until smooth. Slowly add flour mixture and stir until smooth. Add chocolate mixture and combine. In separate bowl beat egg whites until stiff. Fold into batter. Bake in cupcake tins for 20-25 minutes. Remove from oven and cool for 10 minutes in tins. Remove to cool completely. Combine frosting ingredients and frost cupcakes.