In a saucepan, combine sugar and cornstarch. Stir in water until smooth. Cook and stir over medium heat until thickened and bubbly. Reduce heat. Cook and stir 2 minutes longer. Remove from heat.
Stir a small amount of hot filling into egg yolks. Return to pan. Stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from heat. Add lemon juice and butter. Stir until butter is melted and mixture is blended. Pour into baked pastry shell.
MERINGUE: In a saucepan, combine cornstarch, and water until smooth, Cook and stir on medium until thickened, about 2 minutes. Remove from heat. In a mixing bowl, beat egg whites and salt until foamy. Gradually beat in sugar, 1 tablespoon at a time on medium speed until peaks form. Gradually beat in cornstarch mixture 1 tablespoon at a time on high until stiff peaks form. Spread over hot filling.
Bake at 325F for 18-20 minutes. Cool on wire rack.