Dissolve the sugar with 4 tablespoons water in a small pan over a low heat. Increase the heat and boil until it begins to caramelize. Tip in the pistachios and stir, then pour the mixture on to some greaseproof paper on a baking sheet and leave to set.
Put the chocolate in a heatproof bowl. Heat the cream in a pan until it reaches boiling point, then remove from the heat and pour directly over the chocolate, stirring constantly until it has melted. Refrigerate until cold, then beat with a hand-held electric whisk until thick.
Spoon the cold chocolate into serving dishes, decorate with broken shards of the pistachio praline and serve.