Rinse and dry rhubarb, cut into small pieces. Combine sugar with vanilla sugar and about 800 ml (approximately 3 1/2 cup) of water, bring to a boil and simmer until sugar has dissolved.
Add rhubarb to the pan and simmer for about 10 minutes or until rhubarb is soft. Mash coarsely with a fork or press gently with a potato masher.
Dissolve cornstarch in a little cold water. Add to rhubarb soup and bring to a boil again, remove from heat. Serve hot or cold with a dollop of buttermilk or cream.