Grease a 9x9" baking dish with cooking spray.
In a medium pot, melt butter.
Once melted, add the marshmallows and stir until completely melted.
Combine marshmallow mixture and Lucky charms in a large bowl, stirring until evenly distributed.
Press into prepared baking dish and let cool.
Cut into squares and enjoy!
*Note: Keep these in an airtight container. If they become hard, put them in the microwave for 5-10 seconds and this should soften them right up!