Melt butter over medium low heat in a large sauce pan.
Add in the marshmallows and stir together, while melting, until marshmallows are completely melted.
Pour in Lucky Charms cereal and stir until cereal is evenly coated.
Pour cereal treats into a greased 9x13 pan. Carefully press down evenly.
Allow to cool for an hour or two before cutting into shapes.
Microwave white chocolate for 1 minute, stirring after. Microwave for additional 30 second intervals, stirring after each, until creamy.
Dip bottom of cereal bars into melted white chocolate.
Place on waxed paper or parchment and let sit until white chocolate sets.