To the jar of a blender add the chopped watermelon, sugar, juice from the lime and salt. Blend until thoroughly pureed, about 1 minute. Now comes the spice! I recommend adding 1 teeny sliver of cayenne pepper at a time, tasting and then adding if you'd like more. My batch wasn't super spicy--there was just a bit of kick at the finish. If you don't have a cayenne pepper, other alternatives are ground cayenne pepper, Sriracha or chile paste.

Transfer the watermelon mixture to a 8 x 8 dish (or other casserole-like vessel). Cover with plastic wrap and transfer to the freezer. At the 1 hour-mark, stir the mixture, breaking up the icy bits. At the 2 hour-mark, stir once more. At the 3 hour-mark scrape the entire mixture using a fork until flaky crystals form. Cover tightly and freeze. Just before serving, scrap it one more time. Serve with a dollop of whipped cream (it helps with the heat!) and a sprig of mint.