Preheat your oven to 350°F (180°C). Line 2 large baking sheets with parchment paper for easy clean-up and to prevent sticking.

In a large bowl, whisk together the flour, baking powder, baking soda, cornstarch, and kosher salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, add the room-temperature butter, light brown sugar, and granulated sugar. Beat at medium speed for 3-5 minutes, until light and fluffy.

Reduce mixer speed to low and add the eggs one at a time, mixing until just combined. Then, add the vanilla extract.

Gradually add the dry mixture to the wet mixture. Mix until just combined, being careful not to overmix.

Slowly mix in the semi-sweet chocolate chips until evenly distributed throughout the dough.

Using a 2-tablespoon cookie scoop, drop dollops of dough onto the lined baking sheets, flattening each dough ball into a disk.

Taking about a tablespoon of marshmallow fluff, place it in the center of each disk. Place another disk on top and seal the edges.

Place 2 stuffed cookies onto the prepared baking sheet, leaving about 4 inches of space. Bake for 13-14 minutes or until the edges are golden brown.

Allow the cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.