Prepare the syrup by putting the sugar and water in a pan over a low flame. Dissolve the sugar well, then bring to the boil and simmer for 5 minutes. Beat the egg whites with the salt until very stiff.
Pour the hot syrup on to the egg whites and continue beating until it stiffens even further. Add the yoghurt by folding it in with a metal spoon and transfer to a container with a lid. Freeze for an hour, then remove from the freezer and mix again with the electric mixer. Freeze again until needed.