In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugars on medium-high speed for 2-3 minutes. Scrape down sides of the bowl with a spatula.
Add egg and vanilla extract and beat 7-8 minutes.
On low speed, mix in flour, baking powder, baking soda, and salt until just combined (1 minute max.). Don't step away from the machine/over-mix!
Scrape down sides and, still on low speed, paddle in the cornflake crunch and chocolate chips until just incorporated (30-45 seconds), then stir in the marshmallows as well.
Line a cookie sheet with parchment paper. Measure 1/3 cup portions of dough (using a 2 oz. ice cream/cookie scoop) onto the pan. Pat tops of dough mounds flat. Wrap the pan tightly with plastic wrap and refrigerate at least 1 hour and up to 1 week. (Do not bake at room temperature!) *I chilled overnight and baked the next morning.
When ready to bake, preheat oven to 375 degrees and arrange the chilled dough onto parchment/siplat lined baking sheet(s) at least 4 in. apart.
Bake about 18 minutes, until browned around the edges and beginning to brown in the center. Cookies will puff, crackle, and spread. t-mobile site down If they seem pale and doughy on the surface, leave in the oven for about 1 more minute. Cool cookies completely on pan before moving them to a plate or an airtight container. At room temperature, cookies will keep fresh for 5 days-in the freezer, they will keep for 1 month.
