How to Make It
Sprinkle gelatin over 3 tablespoons cold water in a small bowl; let stand 5 minutes to dissolve, stirring once.
Meanwhile, combine heavy cream, sugar, vanilla, and salt in a small saucepan over medium, and cook, whisking often, until sugar dissolves and mixture is hot, 4 to 5 minutes (do not boil). Remove from heat.
Whisk gelatin mixture into cream mixture until fully dissolved. Pour mixture through a fine wire-mesh strainer into a large measuring cup with a spout; discard solids. Stir in buttermilk. Pour mixture evenly into 6 (6-ounce) ramekins or straight-sided glasses (about 1/2 cup each).
Chill, uncovered, until set, at least 8 hours or up to 2 days. Serve in ramekins, or invert onto serving plates. (To invert, run an offset spatula around edges of panna cotta. Dip bottom half of ramekin in hot water for 10 seconds. Invert panna cotta onto serving plate.) Serve with blood orange segments.