Line a square baking dish (9"x 9" / 22cm x 22cm) with baking or parchment paper on all sides.
In the bowl of a food processor combine the butter, sugar, flour, salt and lemon zest. Pulse until the mixture just comes together.
Press the dough into the bottom of the prepared baking dish and flatten with the bottom of a glass or measuring cup.
Bake in a preheated oven for 15-20 minutes or until golden brown. Remove from the oven and set aside while you make the filling.
Whisk together the lemon juice, sugar, eggs, flour and vanilla extract until smooth. Pour onto the crust and place back in the oven to bake for 20-25 minutes or until the filling is set and no longer jiggles when you move the pan.
Remove from the oven and allow to cool for 20 minutes then carefully remove from the pan and allow to cool completely before slicing.
Swiss Meringue topping (optional): Combine the egg whites and sugar in a large glass/metal bowl set over a saucepan of simmering water. Whisk/stir the mixture until the sugar has dissolved. To test, rub a little of the mixture between your fingers. If you feel sugar granules, continue stirring, if it's smooth remove it from the heat.
Using a hand mixer whip the egg whites until they form stiff peaks and the mixture is completely cool.
Transfer to a piping bag then decorate the bars. Torch the meringue with a blow torch and serve.