Break the egg into the flour in a bowl and stir in the milk and water until a smooth batter is formed. Leave to stand for one hour.
Spread the buttered slices of bread with raspberry jam, then sandwich two slices together, pressing down well. Repeat with the remaining slices of bread, then cut out squares or circles from the sandwiches using a pastry cutter.
Melt the butter in a frying pan over a medium heat.
Dip the sandwich shapes into the batter, then add to the pan to fry for 2-3 minutes each side, or until golden-brown all over. (You may have to do this in batches.)
To serve, place onto a large serving plate and sprinkle with icing sugar.