Grandma’s Lemon Meringue Pie

Filling:

1 cup white sugar

2 tablespoons all-purpose flour

3 tablespoons cornstarch

¼ teaspoon salt

1 ½ cups water

2 lemons, juiced and zested

2 tablespoons butter

4 egg yolks, beaten

1 (9 inch) pie crust, baked

Meringue:

4 egg whites

1/2 cup white sugar