Melt 10 oz of small marshmallows and 2 tablespoons of salted butter in a large microwave-safe bowl in 45-second increments.
Stir the mixture every 45 seconds until completely melted.
Stir in 10 oz of Lucky Charms cereal.
Mix until all of the cereal is thoroughly coated in melted marshmallows.
Pour the mixture into a 9x13 baking pan lined with parchment paper.
Press the mixture into the pan using a silicone spatula.
Add extra cereal or rainbow sprinkles on top.
Allow treats to cool for at least 2 hours.
Cut into squares using a butter knife.