Butter four ramekins or individual pudding molds. Place the butter and the broken-up chocolate in a heatproof bowl, place over a saucepan of simmering water and allow the chocolate to melt slowly. Alternatively, cook it in the microwave for approximately 1 minute until melted. Stir and leave to cool. Place the eggs and egg yolks into a bowl, add the sugar and whisk with an electric whisk until pale, thick and doubled in volume. The mixture should leave a trail when spooned. Fold in the chocolate mixture, sift the flour over it and fold in with a large metal spoon. Divide between the four molds and cook for 12 minutes. They should be set but still have a slight wobble. Leave for 1 minute, then run a knife around the edges and turn out onto plates.
Dust with a little cocoa powder and serve immediately.