2 (1/4-oz.) envelope unflavored gelatin (about 2 1/4 teaspoons)
3 tablespoons cold water
2 cups whole milk
3/4 cup granulated sugar
1 vanilla bean pod, split
2 cups buttermilk
1 teaspoon fine sea salt
5 tablespoons apricot jam, peach preserves, or orange marmalade, warmed
Sprinkle gelatin over 3 tablespoons cold water, and let stand 5 minutes to dissolve, stirring once. Meanwhile, heat milk and sugar in a small saucepan over medium. Cook, whisking often, until sugar dissolves and mixture is hot, about 5 minutes (do not boil). Remove from heat; whisk in gelatin mixture until fully dissolved. Let cool slightly, whisking occasionally, about 10 minutes. Scrape seeds from vanilla bean pod into gelatin mixture; discard pod. Gently whisk in buttermilk and salt.
Divide mixture evenly among 8 (12-ounce) straight-sided glasses (about 1/2 cup each). Cover and chill until set, at least 8 hours or up to overnight. Serve in glasses or invert onto serving plates. Spoon about 2 teaspoons jam or marmalade on each serving.