Prepare the panna cotta: Place six 3 1/2-ounce ramekins (3 inches in diameter, 1 1/2 inches deep) on a rimmed baking sheet. Refrigerate until cold. Put 3 tablespoons cool water into a medium bowl. Sprinkle with gelatin; let stand until gelatin has softened, about 5 minutes.

Stir together cream and sugar in a medium saucepan. Heat over medium-high heat, stirring to dissolve sugar, until bubbles form at edges, about 2 minutes. Pour cream mixture into bowl with gelatin; whisk until gelatin has dissolved. Stir in milk. Pour through a sieve into a bowl. Pour cream mixture into chilled ramekins. Refrigerate on baking sheet until set, about 2 hours.

Meanwhile, make the syrup.

To serve, dip one ramekin in a bowl of hot water for 20 seconds; make sure water does not reach the rim. Invert panna cotta onto a dessert plate, gently shaking and tapping ramekin to unmold. If unmolding is difficult, return the ramekin to hot water briefly. Repeat to ummold the remaining ramekins. Drizzle each panna cotta with vanilla syrup, and serve.