Mix the flour, sugar and a pinch of salt in a bowl or food processor. Add the butter and either blitz or rub between your fingertips to create crumbs. Tip in the egg and vanilla and just enough milk to bring the dough together. Knead briefly, then divide the dough in two, with one piece slightly larger than the other. Press into discs, wrap and chill for at least 2 hrs, or overnight. If it’s the latter, remove from fridge 30-40 mins before rolling.

Roll out the smaller disc between sheets of floured baking parchment to form a 25cm disc. Use a shallow 23cm tart tin as a guide, ensuring it’s about 2cm larger, or a 25cm plate. Set aside in the fridge while you roll out the other piece of dough.

Roll out the larger disc so it’s big enough to line the tart tin, pressing it into the indents of the tin and patching as necessary. Put in the freezer while you make the filling.

Heat the oven to 230C/210C fan/gas 8. Combine the yolk and a splash of milk in a small bowl and set aside for glazing later. Tip the sugar into a wide, heavy saucepan, add 100ml water and stir over low-medium heat until the sugar has dissolved. Turn up the heat, bring to the boil, simmer for 2 mins with the lid on. Uncover and boil briskly, swirling the pan but not stirring, for about 10 mins until it caramelises to a pale gold colour. If you have a sugar thermometer, it should read 160C.

Leave to cool for 2 mins, then add the milk with great care (it will bubble up), then the walnuts, butter and a pinch of salt. Stir well, incorporating any sugar crusted round the edge of pan, and boil for 5-8 mins until the mixture thickens slightly and darkens to a pale straw colour (114C on a sugar thermometer).

Put the frozen tart shell, in its tin, on a baking sheet. Pour in the hot walnut mixture, brush the edge quickly with some of the yolk and milk mixture, then slide the chilled pastry lid on top, pressing well to seal (be careful as the filling will still be hot). Brush lightly with more of the glaze, then pierce in centre and put in the oven, reducing the temperature to 200C/180C fan/gas 6. After 15 mins, reduce again to 150C/130C fan/gas 2 and rotate the tart, brush with more glaze, sprinkle with sugar and return to the oven for a further 25 mins until golden brown. Leave to cool completely on a wire rack before removing from tin. Will keep for a week in an airtight container.