Prepare the pan: Line a 9×13-inch baking dish with parchment paper or lightly grease it to prevent sticking. Set aside.

1. Melt the butter and marshmallows: In a large saucepan, melt the butter over medium heat. Add the mini marshmallows and stir until completely melted and smooth. Add vanilla extract if desired. Stir well to combine.
2. Mix with cereal: Remove the saucepan from heat and quickly stir in the Lucky Charms cereal until evenly coated with the marshmallow mixture.
3. Press into the pan: Pour the mixture into the prepared baking dish. Use a spatula or lightly greased hands to press the mixture firmly into an even layer.
4. Cool and set: Let the bars cool at room temperature for 30-60 minutes or until firm and set. Optionally, top with extra marshmallows before they fully set.
5. Slice and serve: Once the bars are cool and set, cut into squares or rectangles, and they are ready to serve!