Mix the rhubarb and sugar, leave to absorb for approx. 1 hr. Drain the rhubarb, retain the juice and set aside.
Place the butter in a bowl, whisk in the sugar, salt, coriander and nutmeg. Add the eggs one at a time and continue to whisk until the mixture becomes lighter in colour. Mix in 3 tbsp of the reserved rhubarb liquid. Mix the flour, wheat starch and baking powder and stir into the mixture. Transfer the batter to the prepared tray. Arrange the rhubarb on top.
Gently beat the egg white and salt. Add the almonds, marzipan, sugar and amaretto, mix roughly, spread on top of the rhubarb.
Approx. 45 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, then leave to cool completely on a rack. Cut the cake into approx. 5 cm squares.