To make the pastry:
1. Place the flour and icing sugar into a bowl and mix well. Add the butter; rub in lightly with your fingertips until the mixture resembles fine breadcrumbs.
2. Add the egg yolk and 2 tbsp water. Stir into the mixture with a round bladed knife adding an extra tbsp of water if required.
3. When the dough begins to stick together, gently knead it into a smooth ball. Wrap the pastry in cling film and chill for at least 30 minutes.
4. Heat the oven to 200°C/Gas 6. Roll out the pastry on a lightly floured surface to about 3mm thick and use it to line a 23cm loose bottom round tart tin, leaving a little excess pastry hanging over the edge.
5. Line the pastry case with a sheet of baking parchment and fill with baking beans. Bake blind for 12–15 minutes, until the pastry is dry to the touch, then remove the parchment and baking beans and return the pastry case to the oven for about 5 minutes until it is very lightly coloured.
6. Use a small, sharp knife to trim away the excess pastry from the edge.
7. Place the pecans on a baking tray and bake for 10 minutes at 160°C to toast lightly and bring out the flavour. Leave to cool.
8. Take 50g of the pecans and chop finely. Increase the oven temperature to 180°C/Gas Mark 4.
To make the filling:
1. Place the chocolate, butter, sugar and treacle in a pan. Heat gently until the butter and sugar have melted.
2. Add the chopped pecans and stir. Leave to cool.
3. Once cooled add the porter, eggs and double cream. Pour into the cooked pastry case. Arrange the remaining pecans in concentric circles to form a pattern on top of the filling.
4. Bake for 30-35 minutes until the filling is risen and just set. Leave to cool in the tin then remove and serve with ice cream or cream.