In a saucepan, whisk together the sugar, flour, and salt.
Gradually whisk in the milk and cook over medium heat, stirring constantly, until the mixture thickens and boils gently.
In a separate bowl, whisk the beaten egg yolks. Slowly add a small amount of the hot milk mixture to the yolks while whisking constantly. This tempers the eggs. Then, pour the egg yolk mixture back into the saucepan.
Continue to cook and stir for another 2-3 minutes until the mixture thickens like pudding.
Remove the pan from heat and stir in the peanut butter, butter, and vanilla extract until smooth and well combined.
Assemble the Pie (5 minutes):
Pour the warm peanut butter custard filling into the cooled pie crust. Smooth the top with a spatula.
Let the pie cool at room temperature for about 30 minutes.
Then, transfer it to the refrigerator and chill for at least 3 hours, or until the filling is completely set.
Whip the Cream (5 minutes):
In a large bowl, use an electric mixer on high speed to beat the heavy whipping cream until soft peaks form.
Add the powdered sugar and continue beating until stiff peaks form.
Garnish and Serve (5 minutes):
Take the chilled pie out of the refrigerator.
Spread the whipped cream evenly over the chilled peanut butter custard.
If desired, garnish with chopped peanuts or chocolate shavings for a touch of texture and flavor.
Slice and serve the pie chilled. Enjoy!