Line a 9x13-inch pan with foil and spray with nonstick cooking spray.

Melt the butter in a large pot over medium-low heat. Add the marshmallows and stir until completely melted. Remove from heat and stir in the vanilla extract.

Add the Lucky Charms cereal to the marshmallow mixture and stir gently until the cereal is fully coated. Pour the mixture into the prepared pan. Press firmly to compact. (Tip: Spray wax paper or your hands with nonstick cooking spray to prevent sticking when pressing down).

Let the treats set until firm (this usually takes about 60 minutes) then slice into bars. Store in an airtight container.