1 envelope unflavored gelatin (about 1 Tbsp.)
3 cups half-and-half
1 cup sugar
2 tsp. vanilla extract
2 cups fresh or frozen blackberries or cherries , plus more for garnish (optional)
Sprinkle gelatin over 2 tablespoons cold water in a small saucepan; let stand a minute to soften. Heat on low, stirring, until gelatin dissolves, 1 minute. Remove from heat.
In a large saucepan, bring half-and-half and 1/3 cup sugar just to a boil over medium-high heat, stirring. Remove from heat; stir in gelatin (it may need to be scraped off pan with a spatula) and vanilla. Ladle into 8 (1/2-cup) ramekins or glasses; cool to room temperature. Cover and chill at least 4 hours or overnight.
Meanwhile, make fruit sauce by cooking berries and 2/3 cup sugar over medium-low heat, stirring occasionally, until berries release their liquid and begin to break down, about 20 minutes. Blend with an immersion blender (or puree in a blender), strain, and chill for 1 hour.
Serve panna cotta in ramekins or on plates: To loosen, dip each ramekin in a bowl of hot water for 3 or 4 seconds, run a knife around the edges, then invert onto a plate. Spoon fruit sauce on top and garnish with extra berries (if using).