Photograph: Tara Fisher
Preheat the oven to 180°C, 160°C fan, gas 4. Line a baking sheet with nonstick baking paper. Whisk the egg white to stiff peaks, then slowly whisk in the sugar to make a thick, glossy meringue.
Put the mixture in a piping bag with a star-shaped nozzle and pipe 12 small swirls on to the baking paper (or 12 small dollops with a teaspoon). Sprinkle over the pistachios and put in the oven. Reduce the temperature to 140°C, fan 120°C, gas 1. Bake for 30-40 minutes, until the meringues are dry on the outside.
Leave to cool on the baking sheet, then carefully remove from the paper. Melt the chocolate in a bowl over a pan of simmering water, remove from the heat and stir in the soured cream until smooth.
Take a meringue and spoon a heaped teaspoon of chocolate on to it. Sandwich with another meringue and place on a board until set. Repeat with the other meringues. Serve once set.
Get ahead: the meringues keep for a week in an airtight box. Sandwich a few hours ahead; chill until 15 minutes before eating.