Add the flour and icing sugar to a large bowl. Add the cold and cubed unsalted butter and rub the mixture together with your fingertips until it resembles breadcrumbs.
Add the egg yolk, and 1 tbsp of cold water into the bowl and mix with your hand or a spatula until the pastry dough comes together - it may take a bit of kneading.
Alternatively, add the ingredients to a food processor and blitz quickly on the pulse setting until the mixture starts to bind, it shouldn't take too long at all
Grease & flour a 9"/23cm loose bottomed tart tin - Roll the pastry mix out onto a lightly floured surface to the thickness of 1-2 coin(s).
Press the pastry into the tin and trim & neaten the edges. Fill any gaps of the pastry with any excess pastry so it's all covered. Refrigerate for 30 minutes.
Preheat the oven to 200ºc/180ºfan - Cover the pastry with baking parchment and fill with baking beans/rice and bake the pastry 'blind' for 15 minutes.
After 15 minutes, remove the parchment and the beans and bake for an extra 5 minutes or until the pastry is cooked through and turning golden!
Once baked, leave to cool as you make the filling.
Filling
In a small bowl, add the plain flour and 50ml of the milk
Mix these together to a paste and so that there are no lumps of flour left
In a medium sized pan, add the butter, and the rest of the milk, and heat until the butter has melted.
Add the light brown soft sugar, and stir to combine and until the sugar has dissolved (low-medium heat)
Once melted, slowly pour in the flour/milk mixture, and keep stirring whilst heating until the mixture has thickened.
Pour onto the tart case, and then set the tart in the fridge for 3-4+ hours
Once set, carefully remove from the tin, and enjoy.
You can use a shop bought pastry instead - still follow the same baking method. If you use a shop bought pastry shell, they are generally thinner and won't fit all of the filling.
You can swap the light brown soft sugar for dark brown soft sugar if you want a deeper molasses flavour