Preheat oven to 350 degrees F. and lightly spray pie dish with cooking spray.
Cream butter and sugars in large mixing bowl. Stir in egg and vanilla. Slowly add flour, pudding mix, baking soda and salt. Mix slightly then stir in M&M’s until well combined. Transfer dough to pie dish and press up sides and bottom. Bake for 40-45 minutes until baked through. As soon as you remove from oven, press center gently with a spoon. Let cool completely.
To prepare peanut butter pie filling stir together the peanut butter, sugar, cream cheese and buttermilk until well combined. Place in center of cookie crust, spreading evenly. Place cream and sugar into stand mixer with stiff peaks form. Sprinkle with additional M&M’s. Refrigerate for 2 hours before serving.