1. Combine butter and chocolate in the top of a double boiler over simmering water. Stir occasionally until melted and mixed thoroughly. Remove pan from double boiler.
2. Using an electric mixer on slow speed, mix in sugar and milk until blended.
3. Return pan to double boiler and continue simmering 20-25 minutes, without stirring, until thickened. (Be sure to have enough water in the bottom of the pan.)