Put sugar and 150 ml water in a pot and simmer for about 15 minutes at medium heat. Wash and stone the cherries. Puree with icing sugar and cherry liqueur in a mixer. Remove sugar syrup from the stove and let it cool down. Mix yoghurt and lemon juice. Stir in the sugar syrup bit by bit.
Whip the cream until stiff and fold it in carefully. Mix yoghurt mass and cherry puree and place in a metal bowl. Place in the freezer for at least 6 hours. Stir several times. Leave to stand at room temperature for about 10 minutes before serving. Use an ice-cream scoop to cut out balls and serve in ice-cream cones if desired.