Preheat the oven to 190°C
Melt the butter in a pan, stir in the flour and cocoa, and blend over a low heat. Add the milk and mix well until thickened slightly. Remove from the heat and set aside to cool, then add the egg yolks and beat in.
Put the egg whites in the bowl, mix on speed 6 for a few seconds, then add the sugar and continue to whisk until soft peaks form. Fold in the sauce.
Pour the mixture into a greased 1.2 litre dish. Stand the dish in a roasting tin filled half full with boiling water, put into the oven, and bake for 40- 45 minutes, until well risen.
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