In a large saucepan, melt the butter over low heat.

Add the marshmallows to the melted butter and stir until completely melted and creamy.

Remove the saucepan from heat and add the Rice Krispies cereal and Lucky Charms cereal, stirring until well combined.

Press the mixture into a greased 9x13 inch pan, using a spatula or wax paper to flatten it evenly.

Allow to cool before cutting into squares.