Beat together the powdered sugar and peanut butter until the mixture is crumbly; set aside. In a large, heavy saucepan, combine 2/3 c sugar and 2 cup milk; heat to scalding or until bubbles start to form on the bottom. Do not let it boil.
Meanwhile, in a medium bowl, beat the egg yolks to mix; blend in 3 T cornstarch, flour and salt. Stir to make a paste. Whisk in the remaining 1 cup cold milk, whisking until the mixture is smooth. Pour in some of the hot milk mixture, stirring to combine.
Add mixture in bowl to the milk in the saucepan. Cook over medium-low heat, stirring constantly, until the mixture bubbles up in the center. Add the butter and 1 t vanilla. Remove from heat and let custard cool. Preheat oven to 350 degrees. Sprinkle 2/3 of the crumbly peanut butter mixture in the bottom of the baked (and cooled) pastry shell. Pour the cooled custard mixture over the top.
In a large mixer bowl, place the egg whites, cream of tartar and remaining 1 teaspoon vanilla. Beat until stiff peaks form. Gradually, while beating, add the 4 tablespoons remaining sugar and 3 T remaining cornstarch. Continue beating until whites are very think and glossy. Spread on top of pie; sprinkle the remaining peanut butter mixture on top.
Bake for 10 to 15 minutes; watching carefully, or until the meringue is golden brown. Cool.