Start by crushing your shortbread cookies into fine crumbs using a food processor or a rolling pin in a zip-top bag.
Transfer the cookie crumbs to a mixing bowl and pour in the melted butter. Stir until the crumbs are evenly coated and have a wet sand-like consistency.
Press the cookie mixture firmly into the bottom and up the sides of a 9-inch pie dish. Ensure it is tightly packed. Place the crust in the refrigerator to firm up.
In a large mixing bowl, beat the cream cheese until smooth and creamy.
Add the peanut butter, granulated sugar, and vanilla extract. Mix until well combined and fluffy, scraping down the sides as needed.
Gently fold in the Cool Whip until the mixture is smooth and no streaks remain, maintaining a light and airy texture.
Remove the crust from the fridge and spoon the peanut butter filling into the crust, spreading it evenly and smoothing the top with a spatula.
Cover the pie with plastic wrap and place it in the refrigerator for at least 4 hours, or overnight, to set properly.
Just before serving, melt the semi-sweet chocolate in a microwave-safe bowl in 20-second increments, stirring until smooth.
Drizzle the melted chocolate over the chilled pie for an irresistible Twix-like topping.
Slice the pie into wedges and serve chilled. Enjoy every creamy, crunchy bite!