Lemon bars

Set the oven to 350 and fully line a 9 by 13 inch baking dish with parchment paper.

Whisk the flour, powdered sugar and salt together in a large bowl.

Pour in the melted unsalted butter, with the vanilla if using, and mix with a spatula.

Work the mixture together by hand, pressing rather than kneading, until no dry flour remains.

Break the shortbread into crumbles, sprinkle it evenly into the lined dish and work it into an even layer with the fingers.

Press the crust down firmly with no gaps or cracks, finishing with a flat bottomed glass and an offset spatula at the corners and edges; gaps let the filling in and float the crust.

Bake the crust at 350 for 20 to 25 minutes, until light golden.

Put the granulated sugar and the lemon zest in a food processor and pulse until the sugar is light yellow and the texture of wet sand; by hand, mince the zest and rub it into the sugar instead.

Juice the lemons to get one cup of fresh lemon juice, with no seeds.

Tip the lemon sugar into a large bowl and whisk in the flour, which thickens and sets the curd.

Add the lemon juice and the eggs and mix until well combined.

Give the filling a final stir and pour it into the hot crust straight out of the oven.

Bake at 350 for about 25 minutes, turning halfway through, until the centre has only a very slight jiggle.

Cool about an hour, then chill in the fridge at least two hours.

Pull gently on the edges to release, lift out by the parchment and dust with powdered sugar through a sifter.

Cut with a dampened knife, wiping it off between each cut.

Store a week in an airtight container in the fridge, or cut, freeze on a baking sheet and keep in a container up to three months.