Preheat oven to 350F
Cream together butter, vanilla, peanut butter, brown sugar, and white sugar
Beat in egg
In a separate bowl, mix together baking powder, baking soda, salt, and flour
Slowly mix dry ingredients into wet, about 1/4 cup at a time, until fully combined. If using GF flour, let rest for about 15-20 minutes
Fold in marshmallow fluff, not mixing too thoroughly so there are streaks of fluff throughout the batter
Line a baking sheet with parchment paper and place small portions of dough onto the pan (the recipe recommends golf-ball sized, but I'd do smaller and end up with more small cookies - they bake better that way)
Bake for about 10 minutes and allow to cool for about 15 minutes before eating