Roasted grapes add a burst of fruity flavor to this silky-smooth Italian dessert.
Place cream and vanilla seeds and pod in a small pot, and bring to a simmer over medium-high heat. Turn off heat, cover, and let steep 10 minutes.
Meanwhile, sprinkle gelatin over water in a small bowl; let soften 5 minutes. Whisk softened gelatin mixture and sugar into cream mixture until sugar dissolves. Pour through a fine sieve into a bowl. Let cool until lukewarm, stirring occasionally. Stir in buttermilk, then pour into a shallow 1-quart baking dish or ceramic pie plate. Refrigerate until set, at least 4 hours or overnight.
Preheat oven to 425 degrees. Place grapes on a parchment-lined rimmed baking sheet, and roast until just bursting, about 10 minutes. Transfer grapes to a wide, shallow bowl, and let cool. Serve panna cotta topped with grapes and any accumulated juices from the bowl.