Beat the egg into the milk with 1 tablespoon (15 g) of the sugar. Cut the bread slices in half and soak in the egg and milk for 5 minutes, changing the top slice to the bottom after 2 or 3 minutes so that they all take up the egg mixture evenly.
Put the cherries in a small saucepan with 2 tablespoons (30 g) of the sugar and heat slowly until almost boiling, then leave the pan over very low heat so that the cherries remain hot but do not continue to cook.
Heat the butter in a large frying-pan until just foaming. Put in the bread and fry on both sides; it should be crisp on the outside and soft in the middle. Quickly place 4 pieces of the fried bread on a warmed dish and, using a perforated spoon, lift a quarter of the cherries on to each. Place the remaining four pieces of bread on top. Sprinkle each hot sandwich with the remaining sugar and serve the cherry juice separately as a sauce.