Add the flour, butter, icing sugar and cinnamon into a food processor and blitz until the mixture resembles breadcrumbs.
Add 2- 3 tbsp water and pulse until the mixture is binded. Cover in cling film and place in the fridge for 20 minutes.
Heat oven to 160°fan/320°F/gas 4. Roll out the pastry on a lightly floured surface and then transfer to a deep 23cm loose-based tart tin.
Leave the excess pastry overhanging the edges of the tin. Place greaseproof paper on top of the pastry, cover with baking beans and place in the fridge to chill for 15 minutes.
Take out of the fridge and bake for 15 mins, remove the paper and beans, and bake for a further 5 minutes until the pastry is crisp and pale golden in colour.
Beat the eggs and sugar together until the mixture resembles a mousse.
Stir in the vanilla and melted butter and then gently fold in the flour and cocoa.
Sprinkle the nuts and chocolate over the pastry case and pour the filling on top.
Place in the oven to bake for 30 minutes until firm but still slightly wobbly.
Trim away the excess pastry and set to one side to cool.
While the cake cools, make your icing. Mix the icing sugar with 1 tbsp of water. Use more water if needed. At the same time melt the chocolate in a bowl over a saucepan of hot water.
Spread a thin layer of icing over your pie once it has cooled completely. Pipe the melted chocolate on top of the icing in a spider web design.