Preheat the oven to 180°C (350°F, gas mark 4).
Mix the flour, baking powder and salt together put in a food processor. Mix in the corn oil using the dough attachment then slowly add the cold water until a dough forms.
Put the dough in a plastic bag and chill for at least 30 minutes.
Roll out the pastry evenly and line a lightly oiled 23cm (8½ in) flan dish. Prick the pastry with a fork, cover with foil and weight with dried beans then bake in the centre of the oven for 15 minutes. Remove the weights and foil and bake the pastry for a further 5 minutes. Set aside to cool. The pastry is now ready to receive a delicious filling. If you're not using the pastry shell immediately, it will freeze for up to 3 months.