Soften Gelatin Powder (1 1/2 teaspoon) with 1 tablespoon cold water in a medium bowl for 5 minutes to bloom.
Meanwhile, put Heavy Cream (1 1/4 cup) and Granulated Sugar (7 tablespoon) into a small saucepan.
Scrape seeds from Vanilla Bean Pod (1/2) into the pan, then add the pod too.
Heat cream over medium heat, stirring until sugar dissolves, about 3-5 minutes, then stir into bowl with gelatin. Stir in Buttermilk (1 3/4 cup), then strain into another bowl.
Divide custard between six 8-oz. ramekins and refrigerate until set, about 3 hours. Meanwhile, make the compote: combine Rhubarb (1.5 pound), Granulated Sugar (1 cup) and zest and juice of the Lemon (1) in heavy saucepan. Stir over medium heat until sugar dissolves.
Reduce heat to medium-low, cover and simmer until rhubarb is tender, stirring occasionally, about 10 minutes. Uncover, stir, and taste. Add Lemon juice and more sugar if necessary. Transfer rhubarb mixture to a bowl of jar.
Cover and chill until cold, about 2 hours. Keep refrigerated. Serve with raspberries or other fruit or rhubarb compote on the side, if you wish.