Lightly grease a 9x13 baking dish with butter, and set aside.
In a Dutch oven or large heavy bottom pan, melt the butter over medium heat.
When about half of the butter is melted, add the marshmallows and stir constantly with a heatproof silicone spatula.
As soon as the marshmallows have melted, pour in the cereal and immediately stir to combine until the cereal is completely coated in the butter marshmallow mixture.
Pour into the prepared baking dish. Use the silicone spatula to pat the cereal down into an even layer in the dish. Allow the bars to set and then cut into squares or rectangles.
Notes
Storage: Store Lucky Charms cereal bars in an airtight container at room temperature for up to three days. Freezer instructions: Flash freeze bars by placing sliced bars on a baking sheet lined with wax paper or parchment paper. Freeze for at least 30 minutes. Remove from the freezer and wrap bars individually with plastic wrap. Place in freezer bags or a freezer safe container with sheets of wax paper in between bars (this makes it easier to remove the bars after they are frozen in the container).