Line a baking sheet (or two) with parchment paper.
In the bowl of an electric mixer or in a large bowl with a hand mixer, combine butter and both sugars and cream togeher for 2-3 minutes. Scrape the side of the bowl and add the egg and vanilla. Continue mixing for about 7 minutes.
Bring the speed of the mixer to low and add the flour, baking powder, baking soda, and salt. Mix until just combined, taking care not to over-mix.
Keeping mixer on low speed, add the Cornflake Crunch and chocolate chips and mix until just combined. Fold in marshmallows.
Using a 2-oz cookie or ice cream scoop, scoop about ⅓ cup of dough onto prepared cookie sheet, leaving about 4 inches of space between each scoop. With your palm, pat the tops of the dough flat.
Wrap cookie sheet tightly with plastic wrap and place in fridge for at least 2 hours and up to a week.
When dough is chilled and ready to be baked, pre-heat oven to 375 degrees. Bake cookies for 9-10 minutes, until the cookies are just beginning to brown. Cool completely on pan before moving to an airtight container.