Start with the Granita: Combine all ingredients into a medium saucepan over medium-low heat & simmer – 10 to 15 minutes.
Cool completely to room temp. Strain into a shallow airtight container; Cover & place in the freezer – 2 hours.
Take a fork & scrape the surface of the granita to form icy crystals (think of a snow cone).
Place back in the freezer – 1 hour. Repeat with taking a fork & scraping the surface to form more icy crystals.
For the panna cotta: Heat the heavy cream, half-n-half & sugar in a saucepan over medium heat to dissolve the sugar; Remove from heat & scrape the seeds from the vanilla bean into the cream; Add the bean pod.
Cover & let infuse – 30 minutes
Sprinkle the gelatin over the cold water in a medium-sized bowl & let stand – 5 to 10 minutes.
Remove vanilla bean from cream mixture & reheat over medium heat; Pour the warm cream over the gelatin; Stir until the gelatin is dissolved.
Divide the Panna Cotta mixture into the 6 ramekins or large wine goblets; Chill until firm - 2 to 4 hours.
Plating: Garnish the panna cotta with a couple of tablespoons of the granita with a fresh rosemary sprig.