Place milk and chocolate chips in a 2-cup glass measurer. Microwave on high for 60 to 90 seconds, until hot, but not boiling. Stir to melt the chocolate chips. In a separate bowl, mix cornstarch and sugar. Blend into the hot mixture. Microwave on full power for 30 to 45 seconds or until mixture begins to boil. Stir every 30 seconds. Chill. Serve.