Preheat oven to 350°F. Line 2 large baking sheets with parchment paper.

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

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

Reduce speed to low. Add eggs one at a time, mixing until combined. Add vanilla extract.

Gradually add dry ingredients to wet mixture, mixing until just combined.

Stir in chocolate chips.

Drop dough by 2-tablespoon scoops onto baking sheets. Flatten into disks. Place about 18 cookies per sheet.

Add about 1 tablespoon of marshmallow fluff to the center of each cookie disk.

Place a second cookie disk on top of the fluff. Seal edges firmly. Repeat with remaining disks.

Bake for 13 minutes or until edges are golden. Cool on baking sheets before serving.