Preheat oven to 375 degrees. Line 2 large cookie sheets with parchment paper and set aside.
Using an electric mixer, beat butter, brown sugar and granulated sugar until creamed, approximately 2 to 3 minutes.
Beat in egg until combined.
Add vanilla and peanut butter then beat until incorporated, scraping down the sides of the bowl as needed. Set aside.
In a separate bowl, whisk flour, baking soda and salt together.
Add dry ingredients to wet ingredients, 1/2 cup at a time, mixing on low speed until just combined. Avoid overmixing or cookies will be dry.
Using a spoon, gently stir in Reese's Pieces, chopped Reese's Peanut Butter cups and chocolate chips.
Using a large 2-Tablespoon sized cookie scoop, scoop dough into balls then place them 2 inches apart on cookie sheets.
Bake for 9 to 10 minutes or until edges are slightly golden and centers are still soft.
Remove cookies from oven. If desired, top with additional candy pieces.
Cool cookies on cookie sheets for 5 minutes then transfer to a cooling rack to cool completely.
Store cookies covered at room temperature for up to a week.