Preheat oven to 375 degrees F (190 degrees C).

In a large bowl cream together the butter and sugar until light and fluffy.

Beat the egg yolks, flour, salt, milk, grated zest, and lemon juice into the creamed mixture.

In a clean bowl and with clean beaters, beat the egg whites until stiff but not dry. Fold the egg whites into the filling. Pour into the unbaked pie shell.

Bake for 15 minutes. Reduce the temperature to 300 degrees F (150 degrees C), and bake for an additional 45 minutes or until the top is golden and a toothpick inserted in the center comes out clean.