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 a 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 incorporated; scrape down the bowl as needed.
Once the eggs are combined, add in the flour/cereal mixture. Mix on low speed until combined. Stir the white chocolate chips and additional cereal.
Before chilling, 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.
When ready to bake, preheat the oven to 350℉. Line your cookie sheet with parchment paper or a silicone baking mat. Place your chilled cookie onto the cookie sheet, spacing them at least 2” apart. Place a few extra white chocolate chips on top of the unbaked dough.
Bake at 350℉ for about 10-12 minutes. Cookies are done when the edges of the cookies look slightly browned.
When the cookies are done baking and are still hot, press in a few marshmallows into the tops of each cookie. Allow the cookies to cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.
Store in an airtight container on the counter for up to five days. Freeze for longer storage.