Take the digestive biscuits and crush into a large bowl. Toast the walnuts and hazelnuts, cool and crush roughly and mix with the biscuits. Put the sugar and cocoa powder in a saucepan, whisk together and whisk in the water. Bring gently to the boil, whisking constantly, remove from heat, whisk in the butter and simmer until of a thick syrup consistency. Set aside to cool for 15 minutes and stir in the vanilla essence. Pour the obtained syrup over the bowl of biscuits and mix till well combined. Press this mixture into a lined 23cm cake tin and allow setting in a refrigerator for 1 hour. Make up a ganache with the fresh cream and chocolate and spread over the cake. Chill again until set. Remove from fridge 15 minutes before serving, garnish with remaining nuts