Soak gelatine in cold water. Peel 2 grapefruits so that the white skin is completely removed. Cut out the pulp between the separating skins, collecting the juice. Squeeze the parting skins. Cut fillets into pieces
Squeeze 2 grapefruits. Measure out 1/4 l juice. Heat some juice and sugar until the sugar is dissolved. Squeeze gelatine and dissolve in it. First stir some yoghurt into the gelatine, then into the remaining yoghurt. Stir in the rest of the juice. Chill the mixture for about 20 minutes until it starts to gel
Whip cream and vanilla sugar until stiff. Fold into the yoghurt mass. Distribute cream and grapefruit pieces alternately in 4 dessert glasses. Chill for at least 4 hours. Decorate with lemon balm if desired.
The yoghurt cream tastes great with oranges instead of grapefruit. But since they are smaller and sweeter, you should rather use 6 oranges and less sugar