1. In a medium saucepan over medium heat, whisk together the milk, heavy cream, and granulated sugar. Heat until warm but not boiling.  

2. In a separate bowl, whisk together the peanut butter, cornstarch, egg yolks, vanilla extract, and salt until smooth.  

3. Slowly pour a small amount of the warm milk mixture into the peanut butter mixture, whisking constantly to temper the eggs.  

4. Return the entire mixture to the saucepan and cook over medium-low heat, stirring constantly, until thickened (about 5 minutes).  

5. Remove from heat and stir in the butter until fully incorporated.  

6. Pour the filling into the pre-baked pie crust and smooth the top.  

7. Let the pie cool to room temperature, then refrigerate for at least 4 hours or until set.  

8. In a mixing bowl, whip the heavy cream with powdered sugar until stiff peaks form.  

9. Spread or pipe the whipped cream over the chilled pie.  

10. Garnish with peanut butter crumbles if desired. Slice and serve!