Pre-heat the oven to 275 degrees.

Pour the cornflakes into a medium bowl and, using your hands, crush until they are about one-fourth their original size. Add the dry ingredients and toss to coat. Add the butter and toss to coat.

Prepare a baking sheet with parchment paper; put aside.

Beat butter and sugars in your stand mixer on medium high for 2-3 minutes. Scrape down the sides, add the egg and vanilla, and beat for 7-8 minutes.

Add the flour, baking powder, baking soda and salt to the mixer - beat on low speed for 1 minute. Scrape down the sides.

Add the cornflake crunch and chocolate chips and beat on low speed for no more than 45 seconds. Add the marshmallows and beat just till incorporated.

Using a 2 3/4 ounce ice cream scoop (or a 1/3 cup measure), scoop the dough out in portions onto your prepared pan (the scoops can be very close together). Pat the tops of the cookies flat and wrap the pan with plastic wrap. Refrigerate overnight, or up to 1 week.

Pre-heat your oven to 375 degrees. On parchment-lined baking sheets arrange the cookies at least 4" apart (I put 4 at a time on each sheet). Bake for 18 minutes, or till browned on the edges and just brown toward the centers.

Cool cookies completely on the sheets before removing. Can be stored in an airtight container for 5 days at room temperature or 1 month in the freezer.