Line a cake tin with baking paper. Cut the chocolate into small pieces and melt in a water bath. Beat the egg yolk with sugar and butter until fluffy. Beat the egg whites until they form peaks. Mix the egg yolk with the melted chocolate and carefully fold in the egg whites. Pour into the cake tin and bake at 185°C for 35 minutes. Leave to cool and tap out onto a cake dish. Sprinkle with cocoa powder.
Tip: this recipe is also wonderful made with nougat chocolate or grated orange peel.