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Melt the chocolate and the butter in a heatproof bowl over a pan of simmering water in a saucepan. Beat the yolks and the sugar together, then add the flour. Add the melted chocolate and butter mixture. Whisk in a third of the beaten egg whites, and then fold in the rest. Pour the contents into the cups and cover with lids. Place in basket no. 3.
Steam for 15 minutes with the starter and the main course.
Remove the covers just before serving. Serve warm or cold.