Preheat the oven to 350° F.
Line your baking sheet with parchment paper or a silicone baking mat. Set aside.
In the bowl of a stand mixer or with an electric hand mixer, cream together the butter, peanut butter, brown sugar and sugar until well combined.
Scrape the sides of the bowl and add in the egg. Mix until combined.
Add the flour, baking soda and salt, mix until the sticky dough forms.
Add the marshmallow fluff in a few separate dollops to the mixer bowl. Mix in short spurts until the marshmallow fluff is marbled through the cookie dough.
Scoop out the dough in 1 1/2 tbsp portions (#40 scoop) and arrange on a parchment lined cookie sheet.
Bake at 350° F for 11-13 minutes. The cookie will spread some and the edges should start to turn golden brown when they are ready to come out of the oven.
Allow the cookies to completely cool on the pan.
Store in an airtight container at room temperature for up to 3 days or in the freezer for up to 3 months. Enjoy!