Pour 0.5 cup of the whole milk into a small bowl and sprinkle the powdered gelatin evenly over the surface. Let stand until fully bloomed and spongy, 5–10 minutes.
In a medium saucepan, combine the heavy cream, remaining 0.5 cup milk, granulated sugar, vanilla bean (pod and seeds), and fine sea salt. Heat over medium until steaming with small bubbles forming at the edges, 3–5 minutes; do not boil. Remove from heat, cover, and let steep 10 minutes for full vanilla flavor.
Remove the vanilla pod. Whisk in the bloomed gelatin mixture until completely dissolved, 1–2 minutes. If needed, return to low heat briefly, stirring, until no grains remain; do not let it simmer. Strain the mixture into a large measuring cup with a spout.
Rinse six 6-ounce ramekins with cold water and shake out the excess (this helps with unmolding). Divide the panna cotta mixture evenly among the ramekins. Let cool at room temperature 10–15 minutes, then cover and refrigerate until set, 4–6 hours, until the centers quiver when gently jiggled.
To unmold, run a thin knife around the edge, dip the bottoms of the ramekins in hot water for 5–10 seconds, and invert onto plates. Serve cold with mixed berries (for serving).