1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a bowl, beat together the butter, peanut butter, brown sugar, and granulated sugar until creamy and well combined.
3. Mix in the egg and vanilla extract until smooth.
4. In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients and mix until just combined.
5. Scoop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them about 2 inches apart.
6. Dollop a small amount of marshmallow fluff on top of each cookie dough ball.
7. Bake for 10-12 minutes, or until the edges are golden brown and the centers are set.
8. Let the cookies cool for 5 minutes on the baking sheet before transferring them to a wire rack to cool completely.

Notes:

• Chocolate Chips: Add chocolate chips for an extra indulgent twist.
• Nutty Crunch: Fold in chopped peanuts or other nuts for a crunchy texture.
• Mini Cookies: For bite-sized treats, reduce dough portions and bake for 8-10 minutes.
• Marshmallow Fluff Center: Make an indentation in the dough and fill it with marshmallow fluff for a gooey center.