Clean, wash and chop the rhubarb. Bring to the boil with 4-5 tablespoons of water and simmer for about 2 minutes. Stir starch and 1-2 tbsp. water until smooth. Stir into the rhubarb, continue to simmer briefly. Season to taste with 3-4 tbsp. sugar. Let cool down for about 30 minutes.
Coarsely crush 6 sponge fingers. Divide into 4 bowls with Amarettini. Sprinkle with Amaretto. Mix quark, mascarpone, vanilla sugar and 2 tablespoons sugar
Spread rhubarb and mascarpone cream on the biscuits. Chill for at least 1 hour. Dust with cocoa. Decorate with remaining lady fingers