Wash and clean the rhubarb and cut it into approx. 0.5 cm thick pieces. Boil up grape juice and vanilla sugar. Add rhubarb and simmer gently for about 5 minutes. Beat the egg white until stiff, adding sugar and salt.
Beat until the mixture is shiny and the sugar has dissolved. Whip the lemon juice. Fold in 1 tablespoon of pistachios. Heat the water in a wide pot, do not let it boil. Using 2 tablespoons dipped in water, pierce 4 large dumplings from the meringue mass and slide into the boiling water.
Put the lid on and let it simmer for 5-6 minutes in the reheat. Remove the dumplings with a skimmer and let them drain. Mix starch and water until smooth and stir into the boiling soup. Let simmer for 1-2 minutes and season with a little sugar if necessary.
Arrange soup and snow dumplings and sprinkle with 1 tablespoon of pistachios.