Method
Make the pastry dough
Put the flour, caster sugar and salt in a large bowl. Rub in the cold diced butter until mostly incorporated, leaving some larger flakes of butter in the mixture.
Mix the apple cider vinegar with the ice-cold water in a jug. Gradually add this to the flour mixture, stirring with a knife or spatula until the dough comes together. Add only as much liquid as you need; if it still feels dry, add extra cold water a tablespoon at a time. The dough should feel slightly softer than a traditional shortcrust pastry, without becoming sticky.
Fold and chill
Turn the dough onto a lightly floured surface and roll into a rectangle about 5mm thick. Fold it into thirds, like a letter.
Roll out and fold twice more, then wrap and refrigerate for at least 30 minutes.
Line the tin
Roll the chilled pastry to roughly the thickness of a £1 coin and use it to line a 23cm pie dish or tart tin.
Trim away the excess pastry and save it for decorations. Return the lined tin to the fridge while you prepare the filling.
Heat the oven
Preheat the oven to 200°C, placing a baking tray inside as it heats. The pie is not blind baked, so the hot tray helps give the base a good start.
Mix the pumpkin filling
Place all the filling ingredients in a large bowl and mix by hand until smooth and evenly combined.
Pour into the chilled pastry case, leaving about 1cm of space below the top edge. Do not overfill; depending on the depth of your tin, you may not need all the filling.
Add the pastry decorations
If decorating, brush the pastry rim with a little milk and attach your pastry shapes around the edge. Brush the shapes with milk and sprinkle with demerara sugar for a little crunch.
Small leaves or pumpkin shapes work well for an autumn pie, while simple spooky cut-outs give it a Halloween finish.
Bake
Carefully place the pie on the hot baking tray. Bake for 10 minutes at 200°C, then reduce the oven temperature to 180°C and continue baking for another 35–45 minutes.
The total baking time is 45–55 minutes. Turn the pie once or twice if needed to help it colour evenly.
The pie is ready when the filling is set around the edges but retains a slight wobble in the centre. It may puff up during baking and will settle as it cools.
Cool and serve
Leave the pie to cool in its tin. Whip the double cream with the icing sugar until softly peaked, then spoon over the cooled pie or serve alongside individual slices.