Mix together flour, cornstarch and sugar in a saucepan. Add 1/4 cup of the milk to the flour mixture and stir until smooth. In another saucepan, scald the remaining 2 3/4 cups milk; add to flour mixture. Cook and stir mixture over medium-low heat; do not allow to boil. In a bowl beat the yolks; remove 1/2 cup hot mixture and add to yolks to temper them; mix well and return to pot. Continue to cook over low heat for an additional 2 to 3 minutes. Remove from heat; add butter and vanilla. Thoroughly fold egg whites into the hot mixture. Place in individual custard dishes or a single bowl, sprinkle with freshly grated nutmeg and chill.