3 large eggs yolks
1 1/2 cups sugar
7 tablespoons cornstarch
1/4 teaspoon salt
1 1/2 cups hot water
1/2 cup fresh lemon juice
2 tablespoons butter or margarine
One 9-inch baked pie shell
Make the filling. In a heatproof bowl, lightly beat the egg yolks to break them up, and then set aside.
In a medium, heavy saucepan, stir together the sugar, cornstarch, and salt. Stir in the hot water until smooth. Bring to a boil over high heat, stirring. When the mixture begins to thicken, reduce the heat to medium-low and cook, stirring, at a very gentle boil for 5 minutes. Remove the saucepan from the heat. Stir several spoonfuls into the yolks. Stir this yolk mixture into the sugar mixture in the saucepan. Cook over low heat, stirring until thickened and bubbly, 3 to 5 minutes.
Remove the saucepan from the heat. Stir in the lemon juice and butter. Place a piece of waxed paper directly on the surface of the filling and let the saucepan cool on a wire rack.
Remove the waxed paper and spoon the cooled filling into the baked pie shell. Serve, or cover and refrigerate for up to a day.