Melt butter and chocolate in the top of a double boiler over simmering water. Stir in remaining ingredients. Cook 20 minutes, stirring occasionally. Let cool slightly and spoon into a (1-pint) glass jar. Refrigerate.
To use, set glass jar in a pot of barely simmering water over low heat until chocolate has thinned to sauce consistency. Stir well. The sauce can be reheated as often as needed. Makes 2 cups.