In a medium bowl, whisk together flour, baking soda, and salt.

In a large bowl, combine butter, peanut butter and brown sugar. Best on medium speed until well combined.

Add egg and vanilla extract. Beat to combine.

Mix together dry ingredients. Add to creamed mixture and beat again to combine until you have a thick dough.

Cover the bowl with plastic wrap and place in the fridge for at least 1 hour.

Preheat oven to 375°F. Line two baking sheets with parchment paper and set aside.

Use a medium cookie scoop and fill it halfway with dough, pressing it into the bottom and up the sides. Spoon a teaspoon of marshmallow fluff into the center, and then top with more dough to fill the scoop. Place each ball of dough onto prepared baking sheets, with no more than 6 on each, evenly spaced out with plenty of room for the cookies to spread.

Bake for 10-12 minutes until the cookies have spread and started to crinkle on top. Baking times will vary per oven temps. Remove from the oven. Carefully move to wire racks to cool completely