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

In a large mixing bowl, whisk together melted unsalted butter and brown sugar until smooth, about 1-2 minutes. Then add the room temperature egg and vanilla extract, whisking for another 2-3 minutes until fluffy.

Gently fold in all-purpose flour, baking soda, salt, and cornstarch. Stop when just combined, then fold in the semi-sweet chocolate chips.

Using a cookie scoop, portion out the dough into balls, flatten each ball slightly, and create a spot for the marshmallow fluff.

Place one tablespoon of marshmallow fluff in the center of each flattened dough disk, and cover with another disk, sealing the edges.

Sprinkle flaked Kosher salt on top and bake for 8-10 minutes until golden brown on the edges.

Let the cookies cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely.