Preheat your oven to 350°F (175°C).

In a large bowl, cream together the butter, peanut butter, brown sugar, and granulated sugar until smooth and fluffy.

Add in the eggs one at a time, mixing well after each addition, followed by the vanilla extract.

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

Gradually mix the dry ingredients into the wet mixture until just combined.

Fold in the marshmallow fluff gently, ensuring it is distributed without fully mixing in.

Using a cookie scoop or tablespoon, drop dough onto a lined baking sheet, spacing them about 2 inches apart.

Bake in the preheated oven for 10-12 minutes, until the edges are lightly golden but the centers still look soft.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring to wire racks to cool completely.