Mix powdered sugar with peanut butter using a fork. Mixture will resemble coarse cornmeal. Press mixture into the baked pie shell, reserving 1Tbsp. for topping.
In a medium saucepan, combine the sugar, cornstarch and salt.
In a separate medium mixing bowl, beat egg yolks and milk together.
Slowly pour the egg mixture into the saucepan whisking both together as you pour.
Cook and stir filling until thick (watch carefully as filling will stick to pan).
Remove from burner and add butter, stir until butter is melted.
Pour the filling into the pie shell being careful not to disturb crumb mixture.
Beat egg whites until almost stiff.
Slowly add sugar and cream of tartar as you continue to beat.
Beat until meringue is stiff.
Top pie with meringue and sprinkle with the reserverd crumbs.
Brown the meringue in a 325 degree oven for 20 mins. or until golden. Serve pie warm or chilled.