In a stand mixer, add the butter, brown sugar, and white sugar. Mix on medium to medium-high speed until light and fluffy, about 2 minutes.

Add in the egg and vanilla. Continue to mix for about 2 more minutes, scraping the sides about halfway through.

Next, add in the peanut butter. Mix on medium speed until peanut butter is incorporated.

Add in your dry ingredients: flour, baking powder, baking soda, and salt. Mix on low speed until the dough has just come together.

Drop the marshmallow fluff over the top of the dough. Use a spatula to fold it into the dough until you see ribbons of white throughout. Don't over mix, so that the streaks of fluff disappear into the dough.

Cover the dough and place into the fridge to chill for at least two hours.

Once chilled, scoop dough onto parchment lined baking sheet. Bake at 350℉ for 9-11 minutes, or until the edges of the cookie are a light golden brown.