Place 1 cup of cereal into a plastic bag and crush with a rolling pin.
In a small bowl, whisk the flour, crushed cereal, baking powder, cornstarch and salt together. Set aside.
Using a mixer fitted with a paddle attachment, cream the butter for about one minute. Add in the sugars and vanilla; mix on medium speed for 1-2 minutes. Scrape down the sides of the bowl.
Add in your eggs, one at a time. Mix until each egg is fully incorporated; scrape down the bowl as needed.
Add in the flour/cereal mixture; mix on low until combined. Stir the white chocolate chips and additional cereal (or mix on low speed). Chill the dough for at least 1 hour. Before refrigerating the dough, I recommend scooping/portioning it first.
Portion the dough with a medium scoop, about 27-29 grams of dough. Place dough on a cookie sheet lined with parchment or use a silicone baking mat. Once all the dough is scooped, cover and refrigerate for 1-2 hours. NOTE: If using a smaller scoop, be sure to adjust your baking time.
When you’re ready to bake, preheat the oven to 350℉. Line your cookie sheet with parchment paper or a silicone baking mat. Place chilled cookie onto the cookie sheet, spacing them at least 2” apart.
Bake at 350 for about 10-12 minutes. Cookies are done when the edges of the cookies look slightly browned.
While the cookies are hot, push in a couple of marshmallows into the tops of each cookie. Let cookies cool on the cookie sheet for about 5 minutes before transferring to a cooling rack.