Melt the butter in a saucepan. Add the cheese, whisky, mustard and a seasoning of salt and pepper. Stir over low heat until the cheese has melted and the mixture is smooth. Remove from the heat and stir in the egg yolk. Add the stiffly beaten egg white and fold evenly through the mixture.
Have the hot toast ready and arrange the slices in the grill pan. Spoon some of the mixture on to each slice of toast and spread evenly. Place under a hot grill and the rarebit mixture will puff up and brown beautifully. Serve at once.