Hot Fudge Sauce

1 can evaporated milk
1 stick butter
4 squares (1 ounce each) unsweetened chocolate
3 cups sugar
1/2 tsp salt

In top of double boiler melt butter and chocolate, cut up. Add sugar and salt. Gradually pour evaporated milk, stirring mixture.
Cook over simmering water for 20 minutes, stirring occassionally