In a small bowl, whisk the flour, ground cereal, baking powder and salt together. Set aside.
Using a mixer fitted with a paddle attachment, cream the butter for about a minute. Add in the sugar 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. Be sure to mix until all of the egg is fully incorporated; scrape down the sides of the bowl as needed.
Add in the flour mixture and mix on low until combined. Stir in (or mix briefly on low speed) the white chocolate chips and cereal.
Before refrigerating the dough, I recommend scooping/portioning it first. For larger cookies, portion the dough with an ice cream scoop. 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 your chilled cookie onto the cookie sheet, spacing them at least 2” apart. Using some extra white chocolate chips, place a couple down onto the top of the cookie. Do the same with a few pieces of dry cereal (not the marshmallows - save those for after baking).
For large cookies, bake at 350 for about 15-18 minutes. For smaller cookies, 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, push in a couple of marshmallows into the tops and let the cookies cool on the baking sheet for about 2-3 minutes before carefully moving them to a wire rack to cool.