Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a medium mixing bowl, whisk together flour, salt, cornstarch, baking powder, and baking soda. Set aside.

In a large bowl, beat softened butter and peanut butter together until smooth and creamy (2-3 minutes).

Mix in granulated sugar and brown sugar, then add egg and vanilla extract. Beat until well combined and fluffy.

Slowly add the flour mixture to the wet ingredients until just combined. Do not overmix.

Cover mixing bowl with plastic wrap and refrigerate for at least 30 minutes.

Scoop about 2 tablespoons of chilled dough, flatten it into a disk, and create a small well in the center.

Spoon about 1 tablespoon of marshmallow cream into the center, fold the dough around it, and roll it into a ball.

Place the cookie dough balls on the lined baking sheet, leaving about 2 inches between each.

Bake for about 10 minutes or until lightly golden brown.

Let cool slightly before transferring to a wire rack. Enjoy!