In a large pan or pot melt butter over low heat. Add in marshmallows and stir until completely melted. Remove from heat. Add vanilla stir until incorporated.
Add the cereal and stir until well coated.
Spray one hand with cooking spray, this prevents the treats from sticking to your hands uncontrollably. Press mixture into 13 x 9 x 2-inch pan coated with cooking spray. Press sprinkles onto the top of the treats. You can incorporate them while mixing in the cereal but it tends to melt the sprinkles.
Cool. Cut into 2-inch squares. Best if served the same day, but will last 2 in an airtight container. You can also freeze the treats for up to 6 weeks.