Butter and line a 900g/2lb loaf tin with cling film. Bring a pan of water to a gentle simmer. Put the sugar and eggs in a heatproof bowl, then place over the simmering water and whisk until pale, thick and doubled in volume. Remove from the heat, plunge the bottom of the bowl into cold water and continue whisking until the mixture is cool.
Melt the chocolate in the microwave on Low, stir, then fold into the egg mixture. Whip the cream to soft peaks and fold into the chocolate egg mixture with the nougat and pistachios.
Pour the mixture into the loaf tin, smooth the surface with a spoon, then cover and freeze until firm. This can be done up to 2 weeks ahead. To serve, turn out onto a serving dish, remove the cling film and leave in the fridge for 15 mins to soften slightly before slicing.
Torrone is a traditional Italian hard nougat and is available in most good delis. This recipe calls for a hard nougat - rubbery pink and white bars won't work here!