1. Cook the sugar, syrup, milk and chocolate, stirring, until the sugar dissolves. Continue cooking to 232° F. (the syrup forms a very soft ball in cold water). Stir occasionally to prevent scorching.
2. Remove from the heat, add the butter and cool without stirring until the bottom of the saucepan feels lukewarm.
3. Add the vanilla and beat until the frosting is creamy and barely holds its shape. Spread quickly on the cake before the frosting hardens.