1. Line a 13 x 9-inch baking pan with foil. Coat foil and a spatula with nonstick cooking spray.
2. Combine marshmallows and butter in a large glass bowl and microwave until melted, about 1 minute. Stir with coated spatula until smooth. Add cereal and stir until well combined. Transfer mixture to prepared pan. With greased hands, evenly pat mixture into pan, pressing firmly. Let cool up to 5 minutes.
3. Combine chocolate and vegetable oil in a medium glass bowl and microwave, stirring often, until smooth, about 60 to 90 seconds.
4. Cut Rice Krispies bar into 24 pieces. Insert an ice cream stick into bottom of each piece. Dip into melted chocolate. Gently tap bottom of stick a few times so that chocolate drips down. Sprinkle with nonpareils. Place on a wax-paper-lined baking sheet.