Grease a 9x13 pan with butter. Set aside.
In a large saucepan or shallow stockpot (non-stick preferably) melt the butter over low heat.
Once the butter is melted, add in 5 cups of the marshmallows, stirring over low heat until they are completely melted and blended with the butter.
Then add in the remaining marshmallows and stir until they are partially melted.
Remove from heat and fold in the Lucky Charm cereal until evenly coated with the marshmallow mixture.
Transfer to the prepared pan and press gently into an even layer. You can use a rubber spatula that has been sprayed with oil or non-stick spray. I find that just wetting my hands and pressing lightly works best & the moisture keeps most of the marshmallows from sticking to my hands.
Add sprinkles if desired and pick out any shaped marshmallows from the cereal to add to the tops. Press in gently.
Let the bars sit at room temperature for at least 2 hours before cutting into squares or rectangles.
Store in an airtight container with small pieces of parchment paper in between each one to keep them from sticking to each other.