Combine the powdered sugar and peanut butter together.
Sprinkle 3/4 of this mixture on the bottom of a baked pie crust. Keep the other 1/4 for topping.
Scald 2 cups of whole milk and set aside to cool.
Combine the cornstarch, sugar, and salt together. Add the slightly cooled milk and mix well.
Beat egg yolks well, add a small amount of the milk mixture over yolks and mix well.
Add eggs to rest of milk mixture. Cook in double boiler until mixture thickens. Remove from heat.
Add the butter and vanilla.
Pour into shell.
Beat the 3 egg whites, cream of tartar, and a pinch of salt until stiff and dry. Slowly add the sugar. Continue beating until stiff. Spread over pie and seal edges.
Place in 325 degree oven on second rack and bake 15-20 minutes (until meringue is browned.)