16 T. room temperature butter
1¼ c. granulated sugar
⅔ cup light brown sugar, tightly packed
½ teaspoon vanilla extract
1½ cup all-purpose flour
½ teaspoon baking powder
½ teaspoon baking soda
1½ teaspoon kosher salt
3 cups Cornflake Crunch
⅔ cup mini chocolate chips
1¼ cups mini marshmallows
In the bowl of a stand mixer fitted with the paddle attachment, combine butter and sugars; cream together on medium-high 2-3 minutes. With a spatula, scrape down sides of bowl, then add egg and vanilla and beat 7-8 minutes.
Reduce speed to low. Add flour, baking powder, baking soda, and salt. Mix just until dough comes together, no longer than 1 minute. Scrape down sides of bowl.
Still on low speed, mix in cornflakes and chips until just incorporated, about 30-45 seconds. Stir in marshmallows.
Line a sheet pan with parchment paper or silpat. Using a 2-oz. ice cream scoop, portion ⅓ cup dough at a time onto pan. Pat tops of dough domes flat. Wrap pan tightly in plastic wrap and refrigerate for at least 1 hour or up to 1 week. (DO NOT BAKE room-temperature disks — they will not hold their shape.)
Heat oven to 375ºF. On a parchment or Silpat-lined sheet pan, arrange chilled disks a minimum of 4 inches apart. Bake 18 minutes, until cookies are browned on edges and just beginning to brown toward the center. Cookies will puff, crackle, and spread. Leave them in oven for another minute or so if they still seem pale and doughy on the surface.
Cool cookies completely on pan before moving them to a plate or an airtight container for storage. At room temperature, cookies will keep fresh for 4 days; in the freezer, they will keep for 1 month.