Place chocolate chips, peanut butter, and butter in a large microwavable bowl,. Microwave, uncovered in 30 second increments until melted, stirring each time. It should be fairly smooth, though thick. Add vanilla and stir. Add cereal, a few cups at a time. Stir carefully to not break apart the cereal. Keep adding cereal until the chocolate mix covers the cereal fairly evenly. Add about half of the powdered sugar and stir, or, as we do, put a good lid on it and shake! Carefully remove lid and stir. You could also place the mixture in a sturdy zip lock bag. Add more powdered sugar as needed. Store in sealed bowl or baggie.