150 g (5 oz) plain dark chocolate (72% cocoa solids), broken into small pieces
4 eggs, separated
100 g (3 1/2 oz) caster sugar
2 teaspoons cornflour
cocoa powder or icing sugar, for dusting (optional)
Grease 6 x 175 ml (6 fl oz) ramekins, then lightly dust with 1 tablespoon of the ground pistachios to cover the base and sides. (This helps the soufflés to rise.) Place on a baking sheet.
Melt the chocolate in a heatproof bowl set over a saucepan of gently simmering water, then leave to cool slightly.
Meanwhile, whisk the egg whites in a clean large bowl with a hand-held electric whisk until stiff, then gradually whisk in half the sugar until the mixture is thick and glossy.
Stir the remaining sugar, egg yolks and cornflour into the cooled chocolate mixture. Gently fold some of the egg white mixture into the chocolate mixture, then gently fold in the remainder, with the remaining pistachios. Spoon into the prepared dishes and spread the tops level, then clean the edges with a fingertip.
Place in a preheated oven, 190ºC (375ºF), Gas Mark 5, for 20 minutes or until risen. Dust with cocoa powder or icing sugar, if using, and serve immediately.