Preheat the oven to 175C/350F/Gas 4.
Place the biscuits into a food-standard plastic bag and crush with a rolling pin or similar heavy object. Tip the crumbs into a mixing bowl.
Add the melted butter and mix well. Press the breadcrumb mixture into the base of a 25cm/10in spring-form cake tin.
Place the cream cheese, ricotta, eggs, caster sugar and vanilla seeds into a food processor and pulse until smooth.
Meanwhile, place the chocolate in a heatproof bowl placed over a saucepan of barely simmering water (don't allow the bowl to touch the water) and leave to melt. Take care not to stir, as this may ruin the chocolate. Once it has all melted, remove the bowl from the heat and allow to cool slightly.
Gently fold the melted chocolate into the cheese mixture, then spoon the cheese and chocolate mixture into the tin, smooth over the top with a palette knife and transfer to the oven. Cook for 50 minutes to an hour, until the surface light and springy to the touch. Remove from the oven and leave to cool.
To serve, carefully release the spring-sides of the tin and remove. To serve, cut out a slice of the cheesecake. Place onto a plate, drizzle the melted chocolate over and serve with a handful of berries and a dollop of cream.