Shake out the elderberry blossoms vigorously, if necessary rinse carefully and allow to drip off. Pour into a bowl, pour champagne over it and let it steep overnight. (If you like an intense taste of elderflower, let the flowers steep in the sparkling wine for up to 24 hours).
Clean, peel and cut the rhubarb into pieces. Weigh 500 g rhubarb and mix with jam sugar in a large pot. Pour the elderberry sparkling wine through a sieve and bring to the boil while stirring occasionally.
Then cook for about 4 minutes while stirring until bubbly. Immediately fill jam to the brim into hot rinsed glasses. Close the jars, turn them over and let them cool down upside down.