In a large bowl, melt the butter in the microwave until it’s almost but not fully melted. Add in the brown sugar and whisk, very well. The butter will be separated from the sugar, whisk it until it’s fully combined.
Crack in the egg and the egg yolk and whisk until the batter is smooth and light colored.
Add the powdered sugar, salt, vanilla, milk powder, baking powder and soda and whisk well.
Add the flour, ground cereal and marshmallow bits and switch to a rubber spatula to mix until fully combined. Cover with plastic wrap and set in the fridge to chill for at least 2 hours.
Preheat the oven to 350 F. Line 2 greased cookie sheets with parchment paper.
Use a cookie scoop to divide the dough, I made big cookies that were about 3 tablespoons each but you can make smaller ones if you like.
With each cookie dough ball, dip face down into the bowl of leftover ground cereal bits pressing to get them to adhere.
These cookies spread so ensure you have enough space between them, at least 1.5 inches between each dough ball. If you are making bigger cookies don’t do more than 5 on a sheet.
Bake for about 12 minutes, until the edges are golden. If you like a bit more crunch on the sides you can bake for another 2 minutes. If you’ve scooped the smaller cookies, check them at the 10 minute mark. Sprinkle on some more cereal/rainbow dust when they come out of the oven.
Store in an airtight container.