In a heavy small saucepan, mix butter and chocolate over low heat to melt.
Remove from heat; add powdered sugar. Gradually stir in evaporated milk. Bring to just boiling; reduce heat. Boil gently over low heat for 8 minutes, stirring frequently. Remove from heat.
Stir in vanilla. Cool slightly. Serve warm over ice cream. Cover and chill any leftovers. Warm over low heat before serving. Makes 1-2/3 cups.