Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Line baking sheets with parchment paper or silicone mats.
In a large mixing bowl, cream together the softened unsalted butter, packed light brown sugar, and granulated sugar until light and fluffy.
3/4 cup unsalted butter, 3/4 cup packed light brown sugar, 1/2 cup granulated sugar
Add the peanut butter, and mix until well combined.
3/4 cup smooth peanut butter
Add the egg and vanilla extract. Then, mix until fully incorporated.
1 large egg, 1 teaspoon vanilla extract
In a separate bowl, whisk together the all-purpose flour, baking soda, cornstarch, and sea salt.
2 1/4 cups all-purpose flour, 1 teaspoon baking soda, 2 teaspoons cornstarch, 1/2 teaspoon sea salt
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Using a 1/4 cup cookie scoop or spoon, portion the dough and roll into balls.
Place the dough balls onto the lined baking sheets, and gently flatten each ball into a disk shape, about 3 inches across.
Spoon approximately 1 tablespoon of marshmallow creme into the center of each dough disk.
3/4 cup marshmallow creme
Carefully bring the edges of the dough up around the marshmallow creme, covering it completely and forming a ball. Ensure the marshmallow is enclosed within the dough. Freeze the dough for at least 30 minutes or refrigerate until ready to bake.
Space the formed dough balls evenly on the baking sheets, leaving room for spreading.
Bake the cookies for 10 minutes, or until the edges are lightly golden. Keep an eye on them to prevent the marshmallow from burning.
Once baked, remove the cookies from the oven and allow them to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.