Tip the flour, butter, icing sugar and cinnamon into a food processor and pulse to make crumbs. Add 2 1?2-3 tbsp water and pulse until it comes together. Wrap in cling film and chill for 20 mins.
Heat oven to 180C/160C fan/gas 4. Roll out the pastry on a lightly floured surface and use to line a deep 23cm loose-based tart tin. Leave the excess pastry overhanging the edges of the tin. Line with baking parchment, fill with baking beans and chill for 15 mins.
Bake for 15 mins, remove the paper and beans, then bake 5 mins more until the pastry is crisp and pale golden.
Beat the eggs and sugar together to create a mousse-like texture. Stir in the vanilla and melted butter, then fold in the flour and cocoa. Scatter the nuts and chocolate over the pastry case and pour the filling on top. Bake for 30 mins until firm with a slight wobble. Trim away the excess pastry and leave to cool. If freezing, wrap in cling film and foil. To defrost, unwrap and thaw.
Serve at room temperature or warm, with a scoop of ice cream and dusting of cocoa.