Line greased pie pan with piecrust. Combine sugar, flour, and salt in a large bowl. Add egg yolks, buttermilk, and melted butter. Mix well. In another bowl, beat egg whites and cream of tartar until stiff. Fold egg whites into buttermilk mixture. Mix carefully and pour into the prepared pie pan. Bake at 375-degrees for 30-35 minutes or until toothpick inserted in center comes out clean.