2) Remove the lid of the food processor and spritz the surface of the mixture thoroughly with water. Replace the lid and pulse 5 times. Add more water and pulse again until mixture holds together when squeezed.
3) Place the pastry in large resealable bag and squeeze together until it forms a ball. Press into a rounded disk and refrigerate for 30 minutes. Place a 20cm metal pie tin in the refrigerator to chill. Preheat the oven to 220C/Gas 7.
4) Remove the pastry from the refrigerator and roll out in the bag until it reaches the edges of the bag and is 25cm to 28cm round. Cut along two sides of the plastic bag and open the bag to expose the pastry on one side. Turn the pie tin upside down on the exposed side. Invert the entire thing and gently pull the plastic bag off the pastry. Press the pastry into the edges and over the lip of the tin and trim off any excess. Place in the refrigerator for 15 minutes.
5) Poke holes around the sides and into the bottom of the pastry. Place a large piece of greaseproof paper on top of the pastry and fill with dry beans. Press the beans into the edges of the tin, set on a baking tray, and bake in the oven for 10 minutes.
6) Remove the paper and beans and continue baking until the pastry case is light golden in colour, approximately 7 minutes longer. Remove from the oven and place on a cooling rack. Allow to cool completely whilst preparing the filling.
For the crumble and filling:
1) Decrease the oven heat to 180C/Gas 4. Place all the crumble ingredients into the bowl of a food processor and process until it forms crumbs. Reserve 50g and set both portions aside.
2) Place the bicarbonate of soda in a medium mixing bowl and pour the boiling water over it. Add the treacle, egg, and vanilla; whisk to combine. Add the larger amount of crumble to the treacle mixture and whisk to combine. Pour this mixture into the prepared pastry case.
3) Sprinkle the remaining crumble mixture evenly over the top of the filling. Place the pie on the middle shelf of the oven and bake for 40 to 45 minutes or until the filling puffs, begins to look dry and starts to crack slightly. Remove from the oven, transfer to a rack, and cool completely before cutting.