Filling: Blend peanut butter with confectioners' sugar until mealy. Sprinkle 2/3 of the mixture over baked pie crust. Combine flour, sugar and salt in the top of double boiler. Stir in milk. Cook over boiling water, stirring constantly until thickened. Stir small amount of cooked filling into egg yolks. Combine with remaining hot mixture; cook for several minutes more. Add butter and vanilla. Pour into pie shell. Top with meringue.
Meringue: Beat egg whites until stiff; add cream of tartar. Mix sugar with cornstarch. Add gradually; beat stiff. Pile on pie; sprinkle with remaining peanut butter mixture. Bake in 400F oven for 8 to 10 minutes or until delicately-browned. Cool before serving.