Place the water, sugar and vanilla in a small pan over a medium heat.
Transfer back to the pan and place over a medium heat, whisking constantly until the crème patissière reaches the consistency of custard. Don't worry if it is lumpy: you can simply sieve the mixture into a bowl and break lumps with the back of a spoon.
Leave to cool at room temperature in a clean bowl and cover with cling film (directly in contact with the crème patissière to avoid a ‘skin’ forming on top.
Add in the melted butter and the flour (sifted).
Cover with the second génoise circle and add a thin layer of marzipan (make it as thin as you can using a rolling pin).