Hot Fudge Ice Cream Topping

In a small saucepan, combine sugar and cocoa. Stir in the milk; add butter. Bring to a boil over medium heat, stirring constantly. Reduce heat; cook and stir for 1 minute or until slightly thickened. Remove from the heat; stir in vanilla. Serve warm. Refrigerate leftovers., To reheat: Place sauce in a small saucepan; bring to a boil, stirring until smooth.