Preheat the oven to 350°F. Line an 9x13-inch baking dish with parchment paper.
In a large bowl, add peanut butter, butter, and brown sugar. Using a hand mixer, combine the mixture until smooth and creamy, about 2-3 minutes.
Scrape down the sides of the bowl. Add egg and vanilla and mix until combined (about 30 seconds).
Add in flour, cornstarch, and salt.
With the mixer on low, incorporate the ingredients until just combined (30 seconds-1 minute).
Spread about half of the dough into the bottom of the prepared baking dish.
Top the dough with the marshmallow fluff.
Using the remaining dough, form small discs and place them over the top of of the marshmallow fluff.
Bake for 35-40 minutes, or until golden.