Mix flour and cornstarch with 1/2 cup cold water in top of double boiler; blend in sugar and salt. Add remaining water; stir until well blended. Bring to a boil over low heat stirring constantly. Cover; cook over boiling water for 15 minutes. Pour about a cup of the hot mixture in the egg yolks, stirring constantly. Then pour the egg yolk mixture into the rest of the hot mixture. Return to double boiler; cook for 5 minutes longer. Remove from heat; add butter and lemon juice. Mix well. Pour into the baked pie shell.
Preheat oven to 350 degrees.
CORNSTARCH MIXTURE RECIPE: Per 4 egg whites, stir 1 tablespoon cornstarch into 1/3 cup cold water, heat while stirring until the starch thickens. Set aside to cool. Beat egg whites until stiff; beat in sugar gradually. After the sugar has been beaten into the meringue and it forms soft peaks, add cornstarch mixture 1 TBSP at a time, beating to stiff peaks. Pile on top of hot pie filling. Make sure the meringue goes all the way to the edge of the pie crust. Bake at 350 for 15 minutes. Remove from oven and allow to cool for 1 hour before putting it in the refrigerator. It must be room temperature before placing it in the refrigerator to keep it from weeping.