Make sure that you have prepared the vegan chocolate pastry cream before starting on the rest of the tart so that it has time to chill. Place the flour, cocoa powder, icing sugar and salt either in a bowl or a food processor and mix. Add the butter and rub in with your fingers or pulse in the food processor until it resembles sand.
Add the cold vodka (if using) and mix to combine. Gradually add enough cold water to bring the pastry together into a ball. Shape the pastry into a disc (don't knead it), wrap and refrigerate for about an hour until firm enough to roll out.
Roll out the chilled pastry on a lightly floured surface until it is large enough to line a 23-25 cm (9-10 in) loose bottomed tart tin. Place the pastry in the tin and gently press it into the corners then trim off the excess.
Prick the base of the pastry shell all over with a fork then place it in the freezer for 20 minutes until solid. Meanwhile preheat the oven to 180°C/160°C fan/350°F/gas mark 4.
Line the pastry case with baking parchment, pressing it right into the corners, then fill with baking beans or dried rice, making sure that the sides are propped up.
Bake for 25 minutes then remove the parchment and baking beans and return the tart shell to the oven for a further 15 minutes until dry to the touch. Set aside to cool.