Buttermilk Panna Cotta with Rhubarb-Strawberry Jelly

INGREDIENTS

• 1 1/4 teaspoons unflavored powdered gelatin
• 1 1/4 cups heavy cream
• 1/3 cup sugar
• 1/4 teaspoons kosher salt
• 1/4 vanilla bean, halved lengthwise
• 1 1/4 cups buttermilk
• 1 cup crème fraîche or sour cream
• 6 ounces strawberries, hulled (about 1 cup)
• 1/4 cup sugar
• 12 ounces rhubarb, trimmed, sliced 1/4" thick (about 3 cups), divided
• 1/2 teaspoons unflavored powdered gelatin