Put the cake into a large serving bowl, or individual glasses and drizzle over the amaretto. Heat the plums, cinnamon and sugar in a large frying pan, cooking gently for 10 minutes until soft. Cool completely then spoon over the cake. Mix the ginger, reserving 1 tbsp, into the yoghurt and spoon onto the plums. Chill until ready to serve. Scatter with reserved ginger and biscuits and serve immediately.