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

In a bowl, beat butter and peanut butter until smooth.

Add brown sugar and mix until combined.

Add eggs one at a time, mixing well after each addition.

In another bowl, whisk together flour, oats, baking soda, and salt.

Gradually add dry ingredients to wet mixture. Mix until just combined.

Stir in M&M candies and chocolate chips.

Using a cookie scoop, place dough onto prepared baking sheets.

Flatten each dough ball slightly and add marshmallow fluff in the center.

Top with mini M&M candies, if desired.

Bake for 15 minutes or until edges are golden.

Cool on baking sheets before transferring to wire racks.