Step #1 Pour milk into medium saucepan; sprinkle surface evenly with gelatin & let this stand 10 mins to hydrate gelatin.
Step #2 Meanwhile, turn contents of two ice cube trays (about 32 cubes) into large bowl; add 4 c cold water.
Step #3 Measure cream into large measuring c or pitcher.
Step #4 With paring knife, scrape vanilla seeds into cream; place pod in cream along with seeds & set mixture aside.
Step #5 Set eight wine glasses or 4-oz ramekins on baking sheet.
Step #6 Heat milk & gelatin mixture over high heat, stirring constantly, until gelatin is dissolved & mixture registers 135 degrees on instant-read thermometer, about 1½ mins.
Step #7 Off heat, add sugar & salt; stir until dissolved, about 1 min.
Step #8 Stirring constantly, slowly pour cream with vanilla into saucepan containing milk, then transfer mixture to medium bowl & set bowl over ice water bath.
Step #9 Stir frequently until thickened to the consistency of eggnog & mixture registers 50 degrees on an instant-read thermometer, about 10 mins.
Step #10 Strain mixture into large measuring c or pitcher, then distribute evenly among wine glasses or ramekins.
Step #11 Cover up baking sheet with plastic wrap, making sure that plastic does not mar surface of cream; put in the fridge until just set (mixture should wobble when shaken carefully), 4 hrs.
Step #12 Serve panna cotta in wine glasses, or, unmold panna cotta from ramekins & serve as soon as possible.
Step #13 To unmold: Pour 1 c of boiling water into a small bowl, count to three, dip a ramekin into the water, & lift the ramekin out of the water.
Step #14 With a moistened finger or a thin knife, press lightly around the periphery of the cream to loosen the edges.
Step #15 Dip the ramekin back into the water for another three-count.
Step #16 Invert a plate on the ramekin.
Step #17 Flip the plate over & let the panna cotta drop on to the plate.