To make the panna cotta filling, place 650ml cream, jam and vanilla in a saucepan over medium heat. Bring to just below boiling point, then remove from heat. Squeeze excess water from the gelatine and stir into the cream mixture until dissolved. Strain into a jug and cool to room temperature.
Place tart shells on a large tray and divide cooled mixture among them. Chill for 3 hours or until set.
Meanwhile, to make the ganache, place dark and white chocolate in separate heatproof bowls. Place remaining 1 cup (250ml) cream in a saucepan over medium heat and bring to just below boiling point. Pour cream into a heatproof measuring jug, then pour 150ml cream over the dark chocolate and 100ml cream over the white chocolate. Stand for 1 minute, then stir each ganache until smooth. Set aside at room temperature for 2-3 hours to thicken to an almost dolloping consistency, stirring occasionally (if they get too thick, soften slightly by setting each bowl over a saucepan of simmering water).
Once panna cotta has set, spoon 1 tbs dark chocolate ganache over half of each tart and 1 tbs white chocolate ganache over the other half. Chill for 1 hour or until set. Serve, or chill until needed.
