Preheat oven to 400 degrees. Bake pie shell according to package directions. Cool completely.
In a bowl, mix powdered sugar and peanut butter together until crumbly. Set aside.
In a heavy saucepan, whisk together milk, granulated sugar, cornstarch, salt, and egg yolks. Heat over medium heat, stirring constantly. Bring to a simmer for a few minutes, until thickened.
Remove from heat and stir in vanilla extract and butter. Stir until all of the butter is melted.
Sprinkle 1/2 of the peanut butter crumb mixture into the pie crust. Pour filling into pie crust.
Sprinkle remaining crumb mixture on top. Cover and chill at least 4 hours before serving.