Pour the cream, milk and vanilla seeds into a saucepan and place the gelatine into a bowl of cold water to soften. Gently bring the cream mixture to the boil.
Meanwhile, pour the sugar into a separate pan and place over a medium heat. Cook, swirling rather than stirring, until the sugar melts and turns a light golden colour. Carefully pour the hot cream into the caramel, stirring constantly, until well combined.
Squeeze the gelatine leaves to drain, then stir them into the warm cream until dissolved. Add a small pinch of salt, then pass the mixture through a fine sieve into 4 moulds, bowls or ramekins – you should have around 125ml per serving. Cover and place in the fridge to set for at least 2 hours.
Preheat an oven to 180°C/ Fan 160°C/ Gas 4. While you wait, halve the blackberries and place in a small pan with the sugar and a squeeze of lemon juice. Cook over a low heat for 5 minutes, or until the berries just start to soften and release their juice. Reserve in the fridge until needed.
Place the hazelnuts on a baking tray and into the oven for 7-10 minutes, stirring regularly, until golden all over. Transfer to a bowl, then crumble in the biscuits and stir in the lemon zest. Cover and set aside until needed.
To serve, boil a kettle and pour some hot water into a deep tray. Dip the base of the panna cotta moulds into the hot water for a few seconds, then turn them out onto plates. The bottoms of the panna cottas should have melted, creating a glossy sauce. Garnish with a pinch of crumble and some blackberries, with a drizzle of the blackberry juices to finish.