Put the amaretto, lemon juice, egg white and 4 tsp of the cherry syrup into a blender (you can also do this in a large cocktail shaker, you will just need to shake it very hard without adding any ice). Whizz up the mixture a few times at a high speed until it is pale and starting to increase in volume. If your blender is suitable for use with ice, add a few handfuls and pulse just twice more to chill the mixture. Alternatively, just stir in the ice until the outside of the jug feels cold. Pour into glasses and garnish with cherries to serve.