In 4-cup measure or small bowl, combine sugars, cocoa, flour and salt.
Stir in milk and syrup, then butter.
Microwave at High 2 to 4 minutes, or until mixture boils; stir well.
Microwave 2 to 5 minutes, or until desired thickness and a rich chocolate color, stirring every 1 1/2 minutes.
Stir in vanilla.
Serve hot or at room temperature.