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 on medium-high speed until smooth, about 1 minute.

Add the peanut butter. Beat for about 2 minutes, or until smooth.

Add brown sugar. Mix on medium-high speed until smooth, about 2 minutes.

Add eggs, one at a time, mixing for 30 seconds after each addition. Scrape down sides of the bowl and beat ingredients together again if necessary.

With the mixer off, add flour, oats, baking soda, and salt. Mix on low until just combined, being careful not to overmix.

Add the M&M's and chocolate chips. Mix until incorporated.

Using a 2-tablespoon cookie scoop, drop dough onto the lined baking sheets. Flatten each into a flat disk. Repeat with remaining dough.

Add about 1 tablespoon of marshmallow fluff onto each dough disk. Top with a second disk of dough and seal edges.

Top each cookie with mini M&M's (optional).

Bake for 15-16 minutes until golden brown. Cool before serving.