Place a baking tray on the bottom shelf of the oven. Preheat the oven to 200°C.
Egg yolk mixture
Heat the butter in a small pan. Add the flour, cook while stirring; the flour must not take on any colour. Remove the pan from the heat. Bring the milk to the boil with the salt and vanilla. Pour through a sieve into the flour, stir to form a paste which separates from the base of the pan, allow to cool slightly. Gradually add the egg yolks, stirring until smooth.
Beat the egg whites with the salt until stiff. Add the sugar and continue beating until the egg whites turn glossy. Beat approx. 1/3 of the egg whites into the egg yolk mixture using a whisk, then carefully fold in the rest using a rubber spatula. Divide between the prepared moulds.
Approx. 20 mins. on a preheated tray on the bottom shelf of an oven preheated to 200°C. DO NOT open the oven, otherwise the soufflé will collapse. Remove from the oven, dust with icing sugar and serve immediately.