Start by heating the cream and sugar in a pan over a medium heat. As soon as bubbles appear, turn down the heat to very low and cook for another 12 minutes. Meanwhile, soak the gelatine leaves in a bowl of cold water for 5 minutes to soften. Add the white chocolate to the hot cream and stir until smooth. Lift the gelatine out of the water (no need to squeeze it) and whisk into the hot cream mixture to dissolve. Remove the mixture off the heat. Grind the pistachios finely in a food processor and add to the cream. Allow the pistachios to infuse for 15 minutes before passing the mixture through a sieve and into a jug. At this stage you can discard the pistachios. Divide the mixture between 4 x 150ml ramekins and chill for 2 hours to set the cream. When you are ready to serve, dip the bases of the moulds very in hot water for a second or two, then invert onto serving plates. Serve the panna cotta with Tuscan biscotti.