Chocolate Cream Filling and Frosting

3 squares semisweet chocolate
2 egg yolks
1 cup sugar
1/2 cup cream
1/2 cup butter
1-2 cups cream, stiffly beaten (optional)

Melt the chocolate in the top of a double boiler. Mix in the egg yolks, sugar and cream. Cook until thick. Remove from the heat and beat in the butter. For a fancier filling, fold in stiffly beaten whipped cream at the last.