Blitz the flour, icing sugar and salt together in a food processor. Add the butter and pulse until a bread-crumb like texture forms. Add the lemon, vanilla and egg and pulse 10 times. The mixture will still be pretty dry and crumbly. Turn out onto a clean surface and gather and press together. Shape into a disc, wrap in cling film and refrigerate for at least 2 hours and ideally overnight.
Rest the lined tart tin in the freezer for at least 30 minutes.