Heat the oven to 180C/160C fan/gas 4 and line two large baking trays with baking parchment. Melt the butter and chocolate together in a heatproof bowl set over a pan of simmering water, then set aside to cool slightly. (Or, do this in short bursts in the microwave.) Beat the egg, egg whites and sugar together in a large bowl for 2-3 mins using an electric whisk until pale and thickened, then beat in the cooled chocolate mixture. Sift over the cocoa powder and fold in gently.

Spoon tablespoons of the mixture onto the prepared trays, spaced well apart (they will spread). Bake for 8-10 mins until set at the edge. Leave to cool completely on the baking trays. Will keep in an airtight container for up to five days.