Grease one 9" x 13" baking dish (or two 8" x 8" pans) with butter, and line it with two sheets of parchment paper cut to size, overlapping each other to provide "handles" on all sides of the baking dish.
Add your marshmallows to a very large microwave safe bowl.
Cube your butter and place it on top of the marshmallows.
Heat the bowl of marshmallows in the microwave on high power for 3 minutes, stirring at the 2 minute mark.
Stir well to combine the marshmallows and butter, and then stir in the vanilla extract.
Add the cereal to the melted marshmallow mixture, and gently fold it it with a silicone spatula.
Gently transfer your lucky charms mixture to the prepared baking dish, and spread it out evenly. Be sure to not pack them in too tightly, as you don't want your treats to be dense.
Allow to cool at room temperature for several hours, OR in the fridge for 1-2 hours.
Once your treats are set, slice them into squares/bars. Serve and enjoy!