Place the marshmallows, butter, and vanilla in a medium-sized saucepan over medium-high heat. Stir the ingredients constantly until all the marshmallows have completely melted and the mixture is smooth.
Turn off the heat and add the cereal to the pan and fold the ingredients together until the cereal is completely coated in the melted marshmallow mixture.
Gently fold in the red, white, and blue sprinkles until they are evenly distributed throughout the mixture.
Spread the mixture into a 9x13-inch baking dish and top with the star sprinkles.
Allow the Rice Crispy Treats to cool in the fridge for 1 hour before cutting them into bars or shapes and serving.
Notes
When stored in an airtight container at room temperature, these treats can stay fresh for up to 3-4 days. However, they are most delicious when consumed within the first day or two after making them.
I find it easier to cut these into squares after they have time to cool. This also makes them less messy to serve (which is a major help if you have little kids).
It is important to stir the mixture constantly in order to prevent marshmallows from sticking to the bottom of your pot.