Preheat the oven to 180°C (160°C fan)/Gas 4.

Line two large baking trays with baking parchment.

Melt the butter and chocolate together in a heatproof bowl over simmering water or in short bursts in the microwave.

Set aside to cool slightly.

In a large bowl, beat the egg, egg whites, and caster sugar with an electric whisk for 2–3 minutes until pale and thickened.

Add the cooled chocolate mixture and beat to combine.

Sift over the cocoa powder and fold in gently.

Spoon tablespoons of the batter onto the prepared trays, spacing them well apart as they will spread.

Bake for 8–10 minutes until set at the edges.

Leave to cool completely on the baking trays before removing.

Store in an airtight container for up to five days.