Bring the cream just to the boil with the cardamom pods, then set aside to infuse for 15 mins.
Keep one square of chocolate aside and melt the rest gently in a large bowl over a pan of barely simmering water. Strain in the cream, then beat with an electric whisk for a couple of mins to thicken slightly. Pour into pots, cover and chill for 4 hrs, or up to 2 days, until cold and set. Very finely grate over the reserved square of white chocolate to serve.