Put the chocolate and butter in a saucepan over simmering water to melt. Beat the egg yolks and sugar until light, and fold in the flour. Whisk the egg whites until they form firm peaks.
Quickly add the melted chocolate to the egg yolk mixture and stir thoroughly.
Fold in the egg whites.
Spoon into buttered soufflé molds dusted with caster sugar.
Bake at 180C for 20 minutes.
To Serve
Allow to cool slightly when cooked, then unmould. Dust with icing sugar and serve with cream or a coffee crème anglaise.