In a small saucepan, stir together sugar, cocoa powder and cornstarch. Add milk and cook, stirring over Mediu heat until thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. Stir in vanilla. Serve the sauce warm or cool. Note: Refrigerate leftove sauce for up to one week. Reheat, uncovered in Microwave on High until warm. Allow 30-60 secones for 1/4 cup, 1/2-1 1/2 minutes for 1/2 cup.