Combine peanut butter and powdered sugar.
Stir with a fork until mixture resembles coarse meal.
Spread mixture in cooled pastry shell; set aside.
Combine 1/2 cup sugar, flour, and salt in a heavy saucepan; gradually add milk, stirring until well blended.
Cook over medium heat, stirring constantly, until mixture is thickened and bubbly.
Remove pan from heat.
Beat egg yolks until thick and lemon colored.
Gradually stir about one-fourth of hot milk mixture into yolks; add to remaining hot mixture, stirring constantly.
Cook over medium heat, stirring constantly, about 4 minutes until thickened.
Remove from heat; stir in butter and vanilla.
Pour over peanut butter mixture in pastry shell.
Combine egg whites (at room temperature) and cream of tartar; beat until foamy.
Gradually add remaining sugar, 1 tablespoon at a time, beating until stiff peaks form.
Spread meringue over hot filling, sealing to edge of pastry.
Bake at 350° for 12 to 15 minutes or until golden brown.
Cool to room temperature.