Butter and lightly flour 4 oven-proof teacups or dariole moulds. Put the butter and chocolate in a bowl set over a pan of simmering water, taking care that the bottom of the bowl does not touch the water, until the chocolate melts. Leave to cool a little.
Use a hand-held electric whisk to beat the eggs, egg yolks and sugar until light and thick. Pour the melted chocolate mixture into the eggs, then quickly stir in with the flour until just combined.
Spoon the batter into the moulds and bake in a preheated oven, 230°C (450°F), Gas Mark 8, for 6–7 minutes; the sides should be just set but the centres still soft. Leave to rest for a moment, then serve in the teacups, dusted with icing sugar, or invert each cake on to a plate, lift away the moulds and dust with icing sugar.