Pre-heat oven to 200C. Combine butter and flour in a small saucepan and warm over medium heat until a roux forms - this should be white and bubbling. Add the milk in three separate equal-sized parts, incorporating each part thoroughly before adding the next, and then again until smooth (this is to ensure no lumps form). Remove from heat and stir in chocolate until melted and combined.

Add egg yolks one at a time, stirring in each yolk until combined before adding the next. Once all are mixed in, leave to cool for 10 minutes.

While it's cooling, whisk the egg whites until soft peaks form, then add the castor sugar and whisk again until soft peaks form. Pour a third of the whites mixture into the chocolate mixture and fold gently in a circular motion until thoroughly incorporated. Fold in the rest of the egg-white mixture until thoroughly incorporated. The mixture should look fluffy and glossy.

Divide between six 180ml ramekins that have been buttered and dusted with more castor sugar. Before placing filled ramekins in the oven, run a knife around the edge to stop them sticking.

Bake for 14 minutes until souffle has risen 1-2cm above the rim of the ramekin. Serve immediately, dusted with icing sugar and with fresh pouring cream. (The souffles should be slightly soft inside.)