Grease a 9x13 baking pan with cooking spray and set aside.
Add the mini marshmallows, salted butter, and vanilla extract to a large microwave-safe bowl, and heat on high for 2 minutes, mostly melting everything.
Remove the bowl from the microwave, and stir the mixture 'til smooth; the mixture will finish melting with stirring.
Quickly stir in the cereal.
Gently spread and pat the mixture into a greased 9x13 baking pan, and allow it to cool. Tip: To prevent your hands from sticking to the cereal mixture, dip your hands in water beforehand, and rub them together; or you can spray them with a light coating of cooking spray.
Use a Christmas tree cookie cutter to cut the treats into trees.
Top each Christmas tree with a mini marshmallow star. It'll stick to the treat with no help needed besides a gentle press of your finger.
Then place the trees on a platter and serve!