Clean, wash and cut the rhubarb into small pieces. Bring the juice and 4 tablespoons of sugar to the boil and steam the rhubarb for about 5 minutes. Stir cornflour and a little water until smooth and stir into the rhubarb. Bring to the boil once and let it cool down.
In the meantime stir mascarpone and yoghurt until smooth. Season to taste with remaining sugar, bourbon vanilla and lemon juice. Arrange cream and rhubarb in dessert bowls. Serve garnished with lemon wheels, lemon balm and wafer rolls