To make the pastry rub the butter into the plain flour and caster sugar using your fingers until it resembles bread crumbs, you could also do this in a food processor
Add the milk a little at a time (you may not need it all, or you may need a little bit more as all flours absorb differently) and bring the mixture together into a dough
Roll the pastry out on a work surface or silicone mat dusted with flour, and line a 9" loose bottomed tart tin with it, trim the edges and prick the bottom all over with a fork
Put the pastry in the fridge to chill for 30 minutes
Meanwhile, pre-heat your oven to 180C Fan/350F/Gas Mark 5
Scrunch up some baking paper, then flatten it out again and place it over the pastry, then add baking beans or some dry rice on top. Bake it for 15 minutes
Take the pastry out of the oven and remove the baking beans and paper. Bake the pastry again for 10 minutes on the same temperature, or until golden and it no longer looks raw on the base
To make the lemon filling place the butter, lemon juice, lemon zest and golden caster sugar in a pan and melt them together on a low heat
In a mixing bowl, whisk together the eggs, egg yolks (save the whites for the meringue) and the cornflour
Once the butter has melted into the lemon juice mixture, pour the mixture slowly into the egg mixture, whisking as you pour
Pour the entire mixture back into the pan and heat it on a medium heat for around 5 minutes until it thickens up, keep stirring it during this time
Once it is thick, sieve the mixture through a fine sieve into a jug, then stir in the double cream
Pour the lemon filling into the pastry case. Put it into the oven on 160C Fan/350F/Gas Mark 4 for 5-8 minutes, until it has a slight wobble to it. Then leave it to cool completely
To make the Italian meringue put the caster sugar and water in a pan, stir them together then bring to a boil. When the mixture reaches 112C (235F), take it off the heat and pour the sugar syrup into a jug
While the sugar water is coming to a boil, in a stand mixer or using an electric whisk, whisk up the egg whites with the cream of tartar until they reach soft peak stage (foamy and they are just starting to holding their shape on the whisk but not quite)
Then, keeping the mixer running on a medium speed, pour in the hot sugar syrup slowly. Be careful not to let the sugar syrup hit the side of the bowl. Turn the mixer speed up to high and mix until the meringue is white and shiny and holds its shape, and the bowl of the mixer feels cool
Dollop the meringue on top of the pie using a spoon and flick it around to give it texture. Finally use a kitchen blowtorch to brown the meringue all over
Serve immediately and store leftovers in the fridge for 2 days