This retro granita dessert recipe reads like a dispatch from the old-school days of recipes scrawled on 3x5 note cards, combining watermelon, white rum, and mint jelly, plus a generous dose of lime juice. The use of mint jelly is ingenious, as it helps the granita have a silkynot grittytexture. The alcohol from the rum will keep the granita from freezing too hard, but its worth keeping an eye on the timing between scrapes. Eat by the pool on a hot day with some tunes from the sixties.