Mix the starch with some juice until smooth. Bring the rest of the juice to the boil and thicken with the mixed starch. Simmer for at least 3 minutes while stirring. Cover the juice surface with cling film and chill for approx. 30 minutes. As soon as the juice is cold, mix yoghurt, sugar and vanillin sugar.
Whip the cream until stiff. Wash the mint and dab dry. Fold the cream into the yoghurt. Stir the juice very gently into the yoghurt, there should be streaks. Divide the yoghurt into 4 large glasses and decorate with mint