Preheat the oven to 350°F. Line baking sheets with parchment paper. Set aside.

In the bowl of a stand mixer with a paddle attachment, beat the butter and peanut butter on medium-high speed until smooth, about 1 minute.

Add the brown sugar and beat for about 2 minutes until smooth.

Add eggs, one at a time, mixing for 30 seconds after each addition. Scrape down the sides and mix again if needed.

In a separate bowl, whisk together flour, oats, baking soda, and salt. Gradually add to the wet mixture on low speed until just combined.

Stir in M&M candies and chocolate chips until evenly distributed.

Using a 2-tablespoon scoop, drop dough onto prepared baking sheets. Flatten slightly with your hand.

Place about 1 tablespoon of marshmallow fluff in the center of each cookie dough disk. Cover with another disk and seal edges.

Top with mini M&M candies if desired.

Bake for 15-16 minutes, until golden edges. Cool on baking sheets before transferring to a wire rack.