In a blender, combine the watermelon, sugar, corn syrup, lime juice, chopped chiles, salt, and piquín chile. Puree until smooth. Taste and, if desired, add more piquín chile. Pour into a container, cover, and refrigerate until cold, at least 2 hours or up to overnight.
Freeze and churn in an ice cream maker according to the manufacturer’s instructions. When the sorbet has finished churning, mix in the reserved watermelon seeds (if using). For a soft consistency, serve the sorbet right away; for a firmer consistency, transfer it to a container, cover, and allow to harden in the freezer for 2 to 3 hours. Serve sprinkled with salted ground chiles, if desired.
Note: If you are using watermelon with seeds and would like to add the seeds to the sorbet, remove the seeds before cutting the melon into cubes, and then mix them in after churning the sorbet. If the seeds are simply left in the flesh, they will break down when the melon is pureed.
Reprinted with permission from Mexican Ice cream, copyright © 2017 by Fany Gerson, published by Ten Speed Press, an imprint of Penguin Random House LLC.
Photography copyright © 2017 by Justin Walker