Use 9 cups of Chex brand cereals, mixed or all one kind. Place in a large bowl. Combine chocolate chips, peanut butter and margarine in a quart microwave safe bowl. Microwave on high for 1 to 1-1/2 minutes or until smooth, stirring after 1 minute. Stir in vanilla. Pour chocolate mixture over cereal, stirring until all pieces are evenly coated. Then pour cereal mix into a plastic zipper-lock bag with the powdered sugar. Seal securely. Shake until all pieces are well coated. Spread on wax paper to cool. Store in an air tight container. Makes 9 cups.