Preheat the oven to 350° F. Line two baking sheets with parchment paper and set aside.
Whisk together the flour, baking powder and salt. Set aside.
Place the butter, peanut butter, light brown sugar and sugar in a large mixing bowl.
Beat until light and fluffy, about three minutes, scraping the bowl down as needed.
Add the egg followed by the vanilla.
Blend in the dry ingredients.
Remove from the mixer and fold in the marshmallow fluff.
Do not overdo, as you want to see the marshmallow fluff rather than have it blended in to the dough.
Use a medium cookie dough scoop (½ tablespoon) to form the balls, placing them on the baking sheets.
Bake at 350°F for 12 minutes, rotating the pans front to back and top to bottom halfway through. Remove from the oven and cool on the pans.