Step 1

Preheat the oven to 350°F and line two large baking sheets with parchment paper.

Step 2

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.

Step 3

Scrape down the sides of the bowl. Add egg and vanilla and mix until combined (about 30 seconds).

Step 4

Add in flour, cornstarch, and salt.

Step 5

With the mixer on low, incorporate the ingredients until just combined (30 seconds-1 minute).

Step 6

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.

Step 7

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.)

Step 8

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

Step 9

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

Step 10

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