Coat an 8 x 8-inch baking pan with cooking spray
Melt the butter in a large bowl in the microwave
Quickly stir in marshmallows and stir thoroughly until melted.
Add cereals and gently fold into melted marshmallow-butter mixture. Stir until cereal is completely coated.
Transfer mixture into prepared baking pan. Lightly press mixture into an even layer.
Cool at room temperature for about an hour.
Cut into 2-inch squares
Melt candy melts, dip tops of each square in melted candy
Sprinkle Lucky Charms cereal on top, allow to cool