Preheat the oven to 180°C fan / 400°F / Gas 6.
Line a baking pan 20x20cm (8x8inch) with baking parchment.
Shortbread biscuit base: In a medium bowl or stand mixer, beat together the butter and sugar until smooth and creamy.
Then add the vanilla, salt and sift in the flour.
Continue beating until the mixture comes together. The mixture will be crumbly.
Pour the dough into your prepared baking pan and press it down into an even layer.
Use a fork to make a few holes in the crust and bake for 10 minutes.
Lemon filling: While the base bakes, place the eggs, sugar, salt, flour, the zest and juice of 2 lemons into a large bowl or measuring jug. Whisk until just combined, the mixture will be thin.
Once the biscuit base is baked either open the oven and pour the filling directly on top of the biscuit base for ease or remove it from the oven and pour the lemon filling over the biscuit base.
Baking: continue baking for 20-25 more minutes or until the centre is slightly firm to the touch and no longer wobbles. Once baked remove from the oven and allow to cool.
Once completely cool, transfer to the fridge to set for 2 hours (or overnight).
Slice the lemon bars and dust with icing sugar before serving.