Place the milk and chocolate chips in a 12 ounce microwave safe mug or 2 cup measure. Microwave on High (100% power) for 60 to 90 seconds or until hot, but not boiling. Stir to melt the chocolate chips. While the milk and chocolate chips are heating, mix the cornstarch and sugar in a separate small bowl. Blend into hot mixture. Microwave for 30 to 45 seconds or until thickened or just begins to boil. Stir often. Cool.