Adapted from Kraft Jet Puffed Marshmallow's recipe for Rice Krispie Treats
Pour marshmallows and butter into microwave safe bowl and microwave on high in 45 second intervals until the butter melt.
Pour in the Lucky Charms and mix well.
Transfer the mix to the greased casserole dish.
Spray hands with cooking spray and press the mixture into the dish and flatten.
Place the tray in the fridge to cool and firm.
Use a shaped cookie cutter to cut out shamrock shapes.
Optional: Sprinkle with green cookie sprinkles.