Chocolate mousse

This recipe is from my childhood, is one of my family's favorite!

Ingredients:
8 oz. Semi-sweet chocolate
8 egg whites
4 egg yolks
6 tablespoons of sugar
1 tablespoon of butter

Directions
Beat the egg whites, set aside. Using a double boiler melt the chocolate until fully melted. Turn off the heat. Next add sugar and egg yolks, mix everything well. Then add the butter. Then slowly add the egg whites and mix well. Put in individual glasses or a bowl and refrigerate. Serve chilled.