Prepare Gelatin Mixture: Sprinkle the gelatin powder over the milk in a small bowl, stirring until the gelatin has fully dissolved. Set aside.
Heat Cream and Sugar: In a saucepan, combine the cream and 100g sugar. Heat over medium heat, but do not bring to a boil. Split the vanilla bean lengthwise and scrape the seeds into the cream. Add the vanilla pod as well.
Combine Gelatin and Cream Mixture: Just before the cream mixture begins to boil, remove the vanilla pod, add the gelatin-milk mixture, and stir over medium-low heat for 1 minute to ensure the gelatin is completely integrated. Strain the mixture through a fine sieve to remove any lumps.
Set Panna Cotta: Lightly grease six ramekins and pour the cream mixture evenly among them. Refrigerate for at least 3 hours, but preferably overnight, to set.
Make Raspberry Sauce: In a saucepan, combine the frozen raspberries, 200g sugar, and the juice of one lemon. Cook over medium heat, stirring and mashing the raspberries until you achieve a sauce consistency with no whole berries remaining. Strain the sauce through a sieve to remove seeds, then chill in the fridge until ready to serve.
Serve: To serve, run a knife around the edges of the ramekins and quickly dip each one in hot water to help loosen the panna cotta. Invert each onto a plate. Spoon some of the raspberry sauce around the panna cotta and garnish with fresh raspberries.