Preheat oven to 200C/400F and grease and line an 8 x 8 inch square pan.
Make the pie crust first: place flour, sugar, salt and butter into your food processor and blitz until you have a breadcrumb like consistency. I would recommend pulsing the mix, not just turning the processor on and leaving it.
Add in water, 1tbsp at a time, pulsing in between until your dough comes together and pulls away from the sides. (You will need to continue pulsing for around 30 seconds after you've added the water) - I needed 4tbsp of water this time round, but have needed 3tbsp in the past. It really depends!!
Alternatively, you can place all, but the water, into a large bowl and rub the butter into the flour mix with your fingers until you have breadcrumb consistency. Then add the water, 1tbsp at a time, mixing with a wooden spoon in between additions until it starts to come together. Then switch to your hands to bring the dough together fully.
Tip dough into your greased and lined pan and push into a level layer, I find it easier to use my hands to push the pie crust into the pan.
Prick the dough all over with a fork and then place a sheet of grease proof paper over the top of it. Tip in ceramic baking beans, or rice, enough to cover the grease proof paper.
Place in the oven for 15-20 minutes, until slightly puffy and the edges are lightly golden. Lightly lift a corner of the grease proof paper to check on the dough. It will be slightly firm to the touch and not shiny and wet looking.
Whilst the crust is baking, make the topping. Place condensed milk, zest, orange juice, caster sugar, vanilla extract and egg yolks into a large bowl and whisk until combined and smooth.
Add in flour and whisk until well combined and there are no lumps. Leave to one side whilst the crust finishes baking.
Once the crust is done, turn the oven down to 180C/350F and lift out the grease proof paper covered in baking beans and tip beans into a heat proof bowl to cool. (You might have to spoon a few off the pie crust yourself, the beans run off everywhere!!!)
Pour your orange topping all over the pie crust and give the pan a little shake to level the topping.
Place in the oven for 17-23 minutes, until lightly golden, slightly darker around the edges, and the middle is set. Wobble it slightly, the middle may move ever so slightly, but it should be set.
Leave to cool in the pan completely, before transferring to a cutting board to slice into 9-12 bars.
Bars will keep in an airtight container, in the fridge for 4 days. Best served at room temperature.