Soak gelatine in cold water. Peel 2 grapefruits so that the white skin is completely removed. Cut out the pulp between the separating skins. Collect the juice.
Squeeze the rest of the grapefruit and measure 1/4 litre of juice. Add sugar and stir until the sugar has dissolved. Squeeze the gelatine and dissolve in a pot at low heat. Add grapefruit juice and mix.
Cold. In the meantime cut the grapefruit fillets into pieces. Whip cream and vanilla sugar until stiff. As soon as the grapefruit juice begins to set, add yoghurt and cream one after the other and fold in.
Fold the grapefruit pieces, except for a few for decoration, carefully into the mixture at the end. Pour into dessert glasses and spread the remaining grapefruit pieces on top. Leave to set in the fridge for about 1 hour.
If desired, serve decorated with lemon balm.