2 cups butter
4 egg whites
1 cup white sugar
1 teaspoon vanilla extract
12 (1 ounce) squares semi-sweet chocolate
Cream the butter until light and fluffy. In a large bowl, beat the egg whites to soft peaks. Add the sugar gradually to the egg whites, beating all the while. Add in the whipped butter. The frosting may clump at this point if the butter is at all chilled. Don't worry, it just means that the frosting will require more beating. Beat until smooth.
Melt the chocolate in a double boiler or the microwave. Cool slightly; the chocolate should still be fairly warm. Beat into the butter frosting.