1. Split the vanilla bean lengthwise and scrape the seeds into a medium saucepan. Add the heavy cream and the vanilla pod.

2. Heat over medium until steam rises and tiny bubbles form at the edges, about 180°F (82°C). Do not boil. Remove from heat, cover, and steep 15-20 minutes.

3. Remove the vanilla pod. Scrape any remaining seeds into the cream.

4. Soak gelatin sheets in cold water for 5-10 minutes until soft. If using powdered gelatin, sprinkle over 2 tablespoons cold water and let bloom 5 minutes.

5. Gently squeeze gelatin sheets to remove water, then stir into warm cream until dissolved. For powdered gelatin, microwave bloomed mixture 5-10 seconds, then stir in.

6. Stir in 1/2 cup sugar and salt until dissolved.

7. Strain the mixture through a fine-mesh sieve into a bowl.

8. Pour into serving glasses. Chill at least 4 hours or until set.

9. For topping, combine blueberries, 1 tablespoon sugar, and lemon juice in a small saucepan. Simmer over medium heat 5-10 minutes until berries break down.

10. For thicker sauce, mix cornstarch and water into a slurry and stir in during the last minute. Cool.

11. Spoon topping over set panna cottas. Garnish with extra blueberries if desired.