**Prepare the Crust:** Ensure your pie crust is fully baked and cooled. Set aside.
**Make the Peanut Butter Crumble:** In a small bowl, mix peanut butter and powdered sugar with a fork until crumbly. Sprinkle half of the crumble evenly over the bottom of the baked pie crust. Reserve the other half for topping.
**Cook the Custard Filling:**
- In a medium saucepan, whisk together milk, sugar, cornstarch, and egg yolks until smooth.
- Cook over medium heat, whisking constantly, until the mixture thickens and begins to bubble, about 8-10 minutes.
- Remove from heat and stir in vanilla extract and butter until fully incorporated.
**Assemble the Pie:** Pour the custard filling over the peanut butter crumble in the pie crust. Smooth the top with a spatula. Allow the pie to cool to room temperature, then refrigerate for at least 4 hours or until fully set.
**Make the Whipped Cream Topping:** Beat heavy cream, powdered sugar, and vanilla extract with an electric mixer on medium-high speed until stiff peaks form.
**Top the Pie:** Spread the whipped cream over the chilled custard filling. Sprinkle the reserved peanut butter crumble on top for garnish.
**Serve:** Slice and enjoy!