Old-Time Buttermilk Pie

Unbaked 9 inch pie shell
2/3 cup sugar
4 Tbsps. flour
1/4 tsp. salt
4 egg yokes
3 tsps. vanilla
2 cups buttermilk
1/4 cup melted butter
4 egg whites

Using a fork, blend together all of the ingredients except the egg whites until smooth. Beat the egg whites until they are stiff but not dry, then slowly beat in the yoke mixture. Turn into the pie shell and bake at 425 for 10 mins. then at 325 for 35 mins. Test by inserting a knife into the center, it should come out clean