Combine the sugar and cocoa in a heavy bottomed medium saucepan. Whisk in just enough buttermilk to form a smooth paste. Whisk in the remaining buttermilk. Cook over medium heat until the mixture simmers and begins to boil, stirring constantly with a wooden spoon, scraping the sides and bottom of the pan. Boil gently for 3 minutes, stirring constantly to prevent burning. Remove from the heat and stir in the vanilla. Cool. Serve at room temperature or slightly warm. Sauce may be stored in the refrigerator up to one week.
NUTRITIONAL INFORMATION PER 2 TABLESPOONS OF SAUCE: 66 calories; 1.2 g fat (14 percent calories from fat); 1.5 grams protein; 15.2 g carbohydrates; .5 mg cholesterol