Preheat oven to 350° with oven rack in the middle position. Line baking sheet with parchment paper and set aside.
In the bowl of a stand mixer using the paddle attachment or with a hand mixer combine the butter and peanut butter on medium speed for 1-2 minutes. Add in the brown sugar and granulated sugar and mix for about another minute. Add in the egg and vanilla extract and mix to combine, about another minute.
In a separate bowl whisk together the flour, baking soda, and salt. Add to the wet ingredients and mix on low speed for 15-20 seconds, until just combined. Add the candy and chocolate chips and use a wooden spoon or a spatula to mix in and evenly distribute throughout the dough.
Using a large cookie scoop or 1/4 measuring cup measure out the dough into balls. Place 1-2 inches apart on prepared cookie sheet. Press each ball down to flatten slightly with the palm of your hand. Top with extra m&ms and chocolate chips if desired. Bake in preheated oven for 12-14 minutes. The edges will be slightly golden and the middle will be set. Allow to cool on the baking sheet for about 5 minutes and then transfer to a wire cooling rack to finish cooling.
Notes
Cookie dough balls can be frozen and baked later from frozen. You may need to add a few minutes to the baking time. Store baked cookies in an airtight container at room temperature for 2-3 days.