• 70 g (2⅔ oz) unsalted butter, softened
• 45 g (1⅔ oz) caster (superfine) sugar
• 25 g (⅘ oz) egg white (about 1 egg)
• 100 g (3½ oz) cocoa, sifted

Preheat the oven to 170°C (325°F/Gas 3). Place the butter and sugar in the bowl of an electric mixer and beat until thick and pale. Add the egg white and beat for 2 minutes, then fold in the cocoa. Spread evenly over a 30 × 20 cm (12 × 8 inch) tray lined with baking paper to 5 mm (¼ inch) thick. Bake for 12 minutes or until evenly coloured. Remove and cool on the tray. Roughly chop and set aside.

This can be made 1 day in advance and stored in an airtight container in the fridge. Bring to room temperature before serving.