Combine brown sugar, flour and salt in top of double boiler, stir in milk gradually. Cook until thick, stirring constantly, remove from heat. Stir small amount of hot mixture into beaten egg yolks; return to hot mixture. Return to heat, cook for 2 minutes longer, stirring constantly. Add butter and vanilla. Pour filling into pastry shell. Preheat oven to 350 Beat egg whites until stiff, beating in sugar gradually. Spread meringue over pie filling, sealing to edge of shell. Bake at 350 degrees for 12 minutes or until meringue is browned.