Sift the flour, salt and sugar into a bowl. Make a well in the centre and beat in the melted butter, milk and egg yolk until the mixture forms a smooth batter. Beat the egg white until it forms stiff peaks, then fold into the flour mixture.

Heat a waffle iron over moderate heat and brush liberally with melted butter. Pour 2 tablespoons of the waffle batter into the iron, close and cook for 2 to 3 minutes on each side or until it has stopped steaming.

Tip the waffle on to a warmed serving plate and keep hot while you cook the remaining batter in the same way, brushing the iron with melted butter between each waffle.

Serve hot.