Clean, wash and cut the rhubarb into small pieces. Squeeze lemon. Boil up rhubarb, lemon juice, cinnamon stick, vanilla sugar and 1⁄4 l water. Cover and simmer for 6-8 minutes until the rhubarb falls apart.
Remove the cinnamon stick.
Stir starch and 5-6 tablespoons of water until smooth. Stir 3 tablespoons of sugar into the rhubarb and continue to cook for about 1 minute. Season to taste with the remaining sugar. Pour the groats into four small bowls and let them cool down.
Coarsely crumble Amarettini. Serve the rhubarb groats with liquid cream and Amarettini.