Line a baking sheet with parchment paper and set aside. Preheat your oven to 350° F. Start your peanut butter sandwich cookies by adding flour, cocoa powder, baking soda and salt in a medium mixing bowl. Stir to combine completely and set aside.
In a large bowl, cream together butter, both sugars and egg until creamy.
Slowly add dry ingredients into your bowl of wet ingredients. Mix this sandwich cookie dough with a spatula until everything is thoroughly combined.
Gently fold Reece’s Pieces into your cookie dough.
Use an ice cream scoop (about ¼ cup in size), scoop mounds of the chocolate sandwich cookie dough on lined baking sheet, about 1-2 inches apart. Bake for 10-12 minutes in the preheated oven and cool completely on a wire rack before assembling.
In a medium-sized bowl, beat the buttercream filling ingredients (butter, peanut butter, icing sugar and milk) with an electric mixer. Start beating on slow speed and work your way up to high speed. Spoon the peanut butter buttercream filling into a piping bag and frost the back of half of your cookies.
Sandwich your cookies by placing a chocolate cookie on top of a buttercream topped chocolate cookie. Serve the chocolate peanut butter sandwich cookies and enjoy!