Preheat the oven to 180°C/350°F/gas mark 4

Line a 20cm/8in square tin with tin foil, making sure the foil goes up & over the sides

Slice one of lemons into 8 wedges lengthwise then slice each eighth into small pieces, making sure to remove the seeds

In a small bowl, mix the slices with the sugar, rubbing the sugar into the lemon with your fingers to help extract the lemon oil

Set aside for about an hour at room temperature

In a medium-sized mixing bowl, combine the flour with the caster sugar, ¼tsp salt, melted butter and vanilla essence

Spoon the mixture into the prepared tin and smooth down with the back of a spoon, making sure it goes right into the corners

Bake for 20-25 minutes, until golden brown

While the base is cooking, finish preparing the filling

Juice the remaining lemon, measure out 3 tbsp of juice and set aside

Using a food processor, blitz the macerated lemon & sugar mixture until smooth

Add the lemon juice, eggs, cornflour, ¼ tsp salt and melted butter

Purée again until the mixture is smooth

Once the base is cooked, remove the tin from the oven and reduce the temperature to 150°C/300°F/gas mark 2

Carefully pour the lemon filling over the cooked crust and return the tin to the oven

Bake for 20-25 minutes or until the mixture is just set

Remove from the oven and allow to cool completely in the pan

Dust with icing sugar prior to serving

Using the edges of the tin foil, lift the cake out of the tin and cut into squares, rectangles or diamond shapes