Put the Amarettini in a freezer bag and crumble finely with a rolling pin. Mix yoghurt, sugar, lemon juice and Amarettini. Halve the melon and remove the seeds with a spoon.
Cut one half of the melon into thin slices, remove the flesh from the skin and cut into pieces. Cut out the flesh of the second half with a ball cutter. Arrange melon balls and slices with yoghurt sauce.
Decorate with lemon balm.