Combine marshmallows and butter in a large glass bowl and microwave until melted, about 1 minute. Stir until smooth. Tint mixture to a bright green with green and yellow food coloring.
Add cereal and stir until well coated. Let cool about 5 minutes. With greased hands, shape about 1/3 cup of the mixture into a small cone, pressing firmly. Decorate cone with red sprinkles. Repeat with remaining cereal and sprinkles. Top each cone with a yellow candy and place on top of a peanut butter cup.