Place the apples, water, lemon juice, vanilla pod and seeds in a pan, bring to the boil, reduce the heat, cover and simmer for approx 10 mins. until soft. Remove the vanilla pod, puree the apples, leave to cool.
Place the butter in a bowl, beat in the sugar and salt. Add the eggs one at a time and continue to beat until the mixture becomes lighter in colour. Stir in the marzipan, lemon zest and apple puree. Mix the almonds, flour and baking powder, add to the bowl and mix, transfer to the prepared tin.
Mix the apples with the reserved lemon juice, spread on top of the cake. Mix the marzipan, flaked almonds and sugar, spread on top of the cake.
Approx. 1 hr. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove from the tin, place on a cooling rack and allow to cool completely. Dust the cake with icing sugar.