Toast the bread in a non-stick frying pan without any oil, divide into 4 ovenproof ramekins (approx. 300 ml each).
Beat together the egg, egg yolk, icing sugar and water using the whisk on a hand mixer for approx. 5 mins. until the mixture is light and fluffy. Thoroughly mix half of the sour milk with the cornflour, stir into the egg mixture.
Beat the egg whites with the salt until stiff, fold the egg whites carefully into the mixture using a rubber spatula, distribute over the bread pieces, top with the grapes.
Approx. 15 mins. in the lower half of an oven preheated to 200°C. Remove the clafoutis from the oven, dust with icing sugar. Serve with the remaining sour milk.