Make your crust: pulse flour and salt in a food processor. Add cold butter and pulse 3-4 times until it resembles crumbs. Add water, 1 tablespoon at a time, pulsing 2-3 times after each addition, until dough forms a ball. Turn out onto plastic wrap and shape into a disk. Chill at least 30 minutes (or overnight).
Note: you can also use a prepared crust from the refrigerator or freezer. If doing so, skip step 1.
Roll out pie crust to 1/8” thick. Place in a 9” pie plate and press to fit. Return to refrigerator until ready to fill.
Preheat oven to 350 degrees.
Cream butter, peanut butter, and both sugars in the bowl of an electric mixer fitted with the paddle attachment, about 2-3 minutes. Add egg, vanilla, milk, and salt and mix until combined. Slowly add flour and mix until dough just comes together. Stir in chocolate chips and peanuts.
Press cookie dough into the prepared pie crust. Top with M&Ms (as many or as few as you’d like!) Cover the edges of the pie crust with small strips of foil (so that it does not brown too quickly). Bake for about 30-32 minutes. Remove foil from pie crust edges and bake an additional 5-7 minutes until crust and the top of the cookie is browned. Cool before slicing. Pie can be made ahead of time and frozen, if desired.