Melt butter in a microwave safe bowl. Add cereal and let sit for 30 minutes so butter will absorbed the cereal flavor. Then chill in the refrigerator until solid.
Lucky Charms Cookies:
When you’re ready to make the cookies, preheat oven to 350°F and line three baking sheets with parchment paper.
Cream butter and granulated sugar on medium speed with a mixer until combined and fluffy. Add eggs; mix until just combined.
Whisk together flour, powdered sugar, baking powder, and salt. Add half of flour mixture to the butter mixture. Mix on low speed until just combined, and then medium speed until completely combined. Repeat with second half of flour mixture and the milk. Stir in cereal. The dough will be a little soft.
Use a 4 tbsp cookie scoop to arrange the cookie dough on the prepared baking sheet, arranging 2-inches apart. (I did 6 cookies on two sheets and 3 cookies on the third.) Press 5 to 6 marshmallows into the top of each ball of cookie dough.
Bake cookies for 8 to 10 minutes, or until crisp around the edges. Let cookies cool on pans for a few minutes before transferring to a cooling rack.