Combine butter and mini marshmallows in a large saucepan. Place over medium-low heat and cook, stirring frequently, until the marshmallows are melted.
Remove the saucepan from the heat. Add vanilla extract and stir to combine.
Pour rice krispies in a large mixing bowl. Pour the marshmallow mixture over the rice krispies and mix together until well combined.
Moisten your hands to prevent the rice krispie treats from sticking. Scoop a portion of the rice krispie treats. The rice krispie treat ball should be just slightly larger than mouth of the ice cream cone. Roll the rice krispie treat into a ball, pressing firmly. Once you have the appropriate size, weigh with a kitchen scale to make the other rice krispie treats the same size.
Place the rolled rice krispie treat onto a baking sheet tray. Immediately sprinkle on nonpareil sprinkles. If you wait too long, the rice krispie treats will dry and the sprinkles will not stick.
Portion and roll the rice krispie treats until all the mixture is used.