Lynne's Hot Fudge Sauce

In a large microwave safe bowl, mix together:
2 cups sugar
1/2 cup cocoa
1 cube margarine or butter

Microwave on low until blended. Slowly add 1 large can evaporated milk. Microwave until it starts to boil, stirring about every minute or so. Once it starts to bubble, boil 4 minutes without stirring. Cool and add 1 tsp vanilla. Serve warm over ice cream.