Grease a 9×13-inch pan and set it aside.

In a large pot, melt ½ cup (113.5 g) salted butter over medium-low heat.

Add 8 cups (600 g) miniature marshmallows to the melted butter and stir frequently until the marshmallows are completely melted and smooth.

Remove the pot from the heat. Immediately add 10 cups (360 g) Lucky Charms cereal and stir gently but thoroughly until the cereal is evenly coated with the marshmallow mixture.

Turn the mixture into the prepared pan. Using a lightly greased spatula or lightly greased hands, press the mixture into an even layer (avoid pressing so hard that the treats become overly compacted).

Allow the treats to set at room temperature until firm, about 30 minutes to 1 hour. Cut into squares and serve.

Store any leftovers at room temperature in an airtight container for up to 4 days.