This is the one that the kids all ask for every time they come home for a visit.
1 cup sugar
6 Tbl cocoa
3 Tbl cornstarch
dash of salt
Mix all of these dry ingredients until incorporated. Stir in 1 cup water. Then add 4 Tbl butter. Cook in microwave for 2 minutes stir, cook 2 minutes, repeat until the right consistency. Add vanilla. Stores well in refrigerator (if there's anything left over).