In a small heavy saucepan melt the chocolate and butter over medium heat. Add the sugar; gradually stir in the evaporated milk; stir to dissolve the sugar. Bring to boiling; reduce heat. Boil gently over low heat for 8 minutes, stirring frequently. Remove from heat. Cool slightly. Serve warm over ice cream. (Cover and chill any leftovers for up to 3 days.) Makes 1-1/2 cups.