Make the Peanut Butter Crumble: In a bowl, mix powdered sugar and peanut butter with a fork or fingers until crumbly. Set aside.
Heat milk, heavy cream, and sugar in a saucepan over medium heat until steaming. In a separate bowl, whisk egg yolks and cornstarch until smooth. Slowly add ½ cup of the hot milk mixture to the eggs, whisking constantly.
Repeat with another ½ cup. Pour the warmed egg mixture back into the saucepan, whisking continuously. Cook until thickened and bubbling (5-7 minutes). Strain through a fine-mesh sieve and let cool slightly.
Assemble the Pie: Sprinkle half of the peanut butter crumble over the bottom of the pre-baked pie crust. Pour the warm custard over it and spread evenly. Top with the remaining crumble.
Chill the Pie: Cover loosely with plastic wrap and refrigerate for at least 4 hours, preferably overnight.
Garnish with whipped cream, extra peanut butter crumble, or a drizzle of chocolate.