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.
To the bowl of a stand mixer with the paddle attachment, add butter, brown sugar, and granulated sugar. Cream at medium speed for 3-5 minutes, or until light and fluffy.
Reduce the speed to low. Add the eggs one at a time, mixing until combined. Mix in vanilla extract.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Mix in the chocolate chips.
Using a 2-tablespoon cookie scoop, drop dough onto the lined baking sheets. Using your hand, flatten each dough ball into a flat disk shape. You should have about 36 cookie dough disks.
Using two spoons (this will keep your hands from getting sticky), add about 1 tablespoon of marshmallow fluff to the center of a disk of dough. (You can also add the fluff to a piping bag and pipe dollops onto the dough disk.)
Place a second dough circle on top of the fluff. Press around the outer edges to firmly seal the disks together. Repeat with the remaining dough disks and fluff. (I had a total of 18 stuffed cookies.)
Place 2 stuffed cookies onto a lined baking sheet, leaving about 4 inches of space between each cookie. (These cookies are large!)
Bake for 13-14 minutes, or until the edges are golden brown.
Let the cookies cool completely on the baking sheets before serving.