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 the sides of the bowl and beat ingredients together again if necessary.

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

With the mixer still on low or by hand, add the M&M's and chocolate chips and mix until incorporated.

Using a 2-tablespoon cookie scoop, drop dough onto prepared baking sheets. Flatten each dough ball into a disk.

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

Place 2-3 cookies on baking sheets, leaving about 4 inches between each.

Bake for 15-16 minutes or until edges are golden brown.

Let cookies cool completely on baking sheets before serving.