For the sauce
Sift the flour and salt into a large bowl, and add the cayenne. Make a well in the flour, and gradually beat in the oil and 175ml/6fl oz cold water to make a smooth batter.
Put the vegetables in a pan of boiling water, and simmer for 5 minutes, then drain well.
Whisk the egg whites until they form peaks, and gently fold them into the flour batter.
Heat the vegetable oil to 170°–180°C/325°–350°F in a deep-fryer (a cube of bread should brown in 30 seconds).
Dip the vegetables into the batter, turning to coat them well. Drain off any excess batter. Fry in batches until golden. Remove with a slotted spoon and drain on kitchen paper.
Meanwhile, put all of the sauce ingredients in a heavy saucepan, and bring to the boil, stirring, until thickened and clear. Serve with the piping-hot fritters.