In large saucepan melt butter over low heat. Add marshmallows and stir until completely melted. Remove from heat.
Stir in Lucky Charms cereal until well coated.
Using a buttered spatula (or wax paper) press mixture into a 13 x 9 x 2-inch pan coated with cooking spray. Allow to cool, then cut into 2-inch squares.
Best if served the same day.