Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper or silicone mats.

In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.

Add the smooth peanut butter, mixing until well combined. Then, add the egg and vanilla extract, and mix until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, baking soda, cornstarch, and sea salt.

Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix.

Scoop and Roll: Use a 1/4 cup cookie scoop to portion the dough and roll it into balls.

Place the dough balls onto the lined baking sheets and gently flatten each ball into a disk shape, about 3 inches across.

Add the Marshmallow: Spoon approximately 1 tablespoon of marshmallow creme into the center of each dough disk.

Enclose the Marshmallow: Carefully fold the edges of the dough around the marshmallow, sealing it completely inside the dough. Freeze the dough for at least 30 minutes before baking.

Bake for 10 minutes, or until the edges are lightly golden. Keep an eye on them to prevent the marshmallow from burning.

Once baked, allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.