Preheat the oven to 350°F and line two large baking sheets 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).

Using a 2-tablespoon cookie scoop, drop dough onto the lined baking sheets. Using your hand, flatten each dough ball into a flat disk shape about 3-inches across.

Using two spoons (this will keep your hands from getting sticky), add about 1 tablespoon of marshmallow fluff to the center of each disk of dough. (You can also add the fluff to a piping bag and pipe dollops onto each dough disk.)

Bring the dough up around the marshmallow fluff, completely covering the fluff and forming a ball. Place on the lined baking sheets.

Bake the cookies for 10-12 minutes, or until the edges are brown. Watch the cookies carefully so that the marshmallow does not burn.

Let the cookies rest for 10-15 minutes before serving.