In a heavy saucepan, combine 1/3 to 1/2 c sugar,1/4 cup unsweetened cocoa powder & 3 Tbsp. cornstarch. Gradually add 2 1/4 c. skim milk. Bring to a boil over medium heat, stirring constantly; boil and stir for 2 minutes. Remove from heat, stir in 2 Tbsp. Promise Activ or other low-fat margarine & 1 tsp vanilla extract. Spoon into serving dishes. Chill until serving or serve warm with fat free half & half (I like Land O’ Lakes) or 0% saturated fat dairy whipped topping). To avoid a skin from forming on top, press plastic wrap directly onto surface when chilling first.