Pour the milk into a medium saucepan, then evenly sprinkle the gelatin over it. Let it stand to hydrate for about 10 minutes.
Scrape the seeds from the vanilla bean and add them to the cream in a large measuring cup. Set aside.
Arrange eight 4-ounce molds or ramekins on a baking sheet and set them aside. Prepare a large bowl of ice water by combining 2 trays of ice cubes with 4 cups of cold water; set it aside.
Heat the milk and gelatin mixture over high heat, stirring constantly, until the gelatin is dissolved and the mixture reaches 135 degrees Fahrenheit on an instant-read thermometer, about 1½ minutes. Do not over boil the gelatin*. Remove from heat and stir in the sugar and salt until dissolved, about 1½ minutes. Slowly add the cream and vanilla mixture, stirring constantly.
Strain the mixture into a medium bowl and gently set the bowl into the ice water. Chill the mixture, stirring frequently, until it thickens to the consistency of eggnog and reaches 50 degrees Fahrenheit on an instant-read thermometer, about 10 minutes.
Using a ladle, pour it evenly into the molds or ramekins, leaving a small space at the top. Refrigerate until just set and chilled, at least 4 hours or up to 3 days.
When ready to serve, if using a ramekin, run a thin knife between the panna cotta and the side of the ramekin in one smooth stroke. If using a mold, and the shape makes this difficult, quickly dip it into a hot water bath to loosen the panna cotta. Flip the mold/ramekin upside down onto individual serving plates and gently shake to unmold the panna cotta. Lift the mold/ramekins from the plates.
Garnish each panna cotta with a piece of rhubarb, rhubarb syrup, and a light drizzle of lilac syrup. Serve right away.