1/2 cup sugar
1/2 cup brown sugar
3 Tbs. cocoa
2 Tbs. flour
1/8 tsp. salt  
Mix all together in microwave safe bowl, then add
1/3 cup milk  and microwave 2-4 minutes until it boils (watch carefully).
Then add
1/3 cup more milk
2 Tbs. light corn syrup
1 Tbs. butter or margarine  and stir well. 
Microwave 2-5 minutes, stirring every minute until desired thickness.  It will continue to thicken as it cools.  Watch carefully, as it has a tendency to boil over.  Stir in 1/2 tsp. vanilla.  You can keep it warm in a little crock pot if you like.