In a large saucepan, combine sugar and cornstarch. Gradually add milk and water, stirring constantly. Stir in egg substitute. Over medium heat, heat mixture to a boil, stirring constantly. Reduce heat to low; cook 5 minutes, stirring constantly. Remove from heat and stir in vanilla. Pour into an 8 x 8 x 2-inch metal pan. Freeze until ice crystals form around the edge of pan, about one hour. Spoon into a large bowl. Beat at high speed of electric mixer until smooth. Return to pan. Repeat freezing and beating step once. Freeze until firm. Serve immediately.