Combine sugar, flour, buttermilk, egg yolks, and lemon juice in top of a double boiler. Cook mixture over boiling water, stirring constantly, 30 minutes or until thickened and smooth. Remove from heat. Pour filling into pastry shell.
Beat egg whites (at room temperature) until stiff peaks form. Spread over filling, sealing to edge of pastry. Bake at 425° for 8 minutes or until lightly browned. Cool. Refrigerate until chilled.