Photo: Tara Fisher
Preheat the oven to 180°C/fan 160°C/gas mark 4. Line a baking sheet with non-stick baking paper. Whisk the egg white to form 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. Sprinkle over the pistachios and put in the oven. Reduce the temperature to 140°C/fan 120°C/gas mark 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.
Spoon a heaped teaspoon of chocolate on to a meringue. Sandwich with another meringue and place on a board until set. Repeat with the other meringues. Serve once set.