Beat the yolks in a good sized basin until light and creamy. Add the sugar and a very little salt. Scald the milk and pour it gradually over the yolks, stirring constantly whilst doing so. Pour the custard back into a double-boiler in the outer part of which is very hot water. Stir continually until the custard thickens. When it is thick enough it will coat the spoon. It must not boil or the custard will curdle. Strain at once into a jug. Add the flavouring when cool.