In a medium bowl add flour, baking soda, and salt; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugars until mixture is light and fluffy, about 2 minutes.
Add egg, vanilla and peanut butter and mix until well blended.
Add flour mixture, and beat until fully combined.
Fold peanut butter chips into cookie dough.
Add dough to a 10-inch ovenproof skillet, and press to cover the bottom fully.
Bake 40-45 minutes, until edges are brown.
Be sure not to over bake. The cookie will continue to cook in the heavy skillet after it’s removed from the oven.
Transfer to a wire rack to cool, 15 to 20 minutes.
Place marshmallow fluff in microwave safe bowl. Warm in microwave 15-20 seconds, remove and stir fluff quickly. Marshmallow fluff should be a spreadable consistency.
Spread warm marshmallow fluff onto the top of the cookie. Be careful to lightly spread the warm fluff. Like icing a cake, if you press too hard, you will pick up crumbs.
Top the marshmallow fluff with crushed peanuts while it’s sticky. Don’t worry though if your peanuts don’t stick immediately to the fluff. Just press the peanuts down into the marshmallow to keep them from falling off the cookie.
Cut the cookie into wedges and serve warm.
Recipe by In The Kitchen With KP at https://www.inthekitchenwithkp.com/recipes/fluffernutter-skillet-cookie