Put your unopened tins of condensed milk in a high-sided pan, covered with water. Bring to the boil, then reduce the heat and simmer constantly for about 3 hours with a lid on top. It’s very important to remember to keep checking the pan, as you don’t want it to boil dry – otherwise the tins will explode. It will give you the most amazing toffee. Put the tins to one side and allow to cool.
First of all you need to make your pastry. Score down the length of the vanilla pod, if using, and remove the seeds by scraping a knife down the inside of each half (keep the pod for making vanilla sugar). Cream together the butter, icing sugar and a small pinch of salt and then rub in the flour, vanilla seeds, zest of the lemon and egg yolks – you can do all this by hand or in a food processor. When the mixture looks like coarse breadcrumbs, add the cold milk or water. Pat and gently work the mixture together until you have a ball of dough, then flour it lightly and roll it into a large sausage shape – don’t work the pastry too much otherwise it will become too elastic and chewy, not flaky and short as you want it to be. Wrap the dough in clingfilm and place in the fridge to rest for at least an hour. Remove it from the fridge, slice it up and line a 28cm/11 inch tart mould with the slivers. Push them together, then tidy up the sides by trimming off any excess. Place the tart mould into the freezer for an hour.
Preheat the oven to 180ºC/350ºF/gas 4. Take the pastry case out of the freezer and bake blind in the preheated oven for 15 minutes. Peel and quarter the apples and remove the cores, then slice finely and toss in the icing sugar. Remove the pastry base from the oven and smear the caramel from both tins of condensed milk over it. Place the apples on top and pour any remaining juices over. Cook at the bottom of the preheated oven for about 40 minutes, to give you a crispy base and bubbling toffee over the apples. Serve with vanilla ice cream. Beautiful!