Line a baking sheet with parchment paper and set aside.
In a medium bowl, whisk the all-purpose flour and baking powder together and set aside.
In a large mixing bowl, beat butter and the sugar until well mixed.
Mix in peanut butter until well incorporated
Add the egg and mix until well blended.
Add the flour mixture into the wet ingredients and mix until well incorporated.
Roll dough into 2 inch balls and place on the prepared baking sheet.
Flatten each dough ball with your hand, or bottom of a glass, and place up to 1 tablespoon of marshmallow fluff in the middle.
Cup the flattened dough ball in your hand and roll it back into a ball with the marshmallow fluff in the middle.
Place the filled cookie dough balls on the parchment covered baking sheet.
Bake the cookies for 15 minutes or until golden brown.
Cool the cookies slightly, then remove them from the baking sheet. Enjoy!
Notes
If using a glass to press the cookie dough flat, dip the glass into a little bit of sugar before you press the cookie flat, this will help to keep the glass from sticking to the dough.If you do not have marshmallow fluff, try using mini marshmallows or a standard-sized marshmallow. Just note that the texture might be slightly different.