The coarse crystals in this tart and herby granita distinguishes it from other frozen desserts.
Bring sugar and 1 cup water to a simmer in a small saucepan over medium-low heat, stirring until sugar has dissolved. Remove from heat. Stir in basil. Let syrup stand 15 minutes.
Stir together orange juice, vanilla, and salt in a medium bowl. Pour basil syrup through a fine sieve into the orange juice mixture; discard basil.
Transfer mixture to a 9-by-13-inch shallow nonreactive dish. Chill in freezer until edges are frozen, about 1 1/2 hours. Remove from freezer; scrape with fork tines, pulling from edges into center. Return to freezer. Repeat process every 30 minutes until mixture is the texture of shaved ice, about 4 1/2 hours. Spoon into serving bowls, and garnish with basil.
For the right texture, scrape the granitas every half hour for 4 1/2 hours. Same-day preparation is best -- after too long the consistency will change from light and layered to overly dense.