Preheat your oven to 190°C/375°F. Line your pan with parchment paper so you have a sling (this will make it easier to take the bars out) and butter the bottom and sides.
Pulse your graham crackers in a food processor until fine. Pour the melted butter, light brown sugar and salt and pulse until it is completely moistened.
Pour the crust into the bottom of the pan and use your fingers, the bottom of a metal measuring cup or a silicone spatula to press gently into the bottom. Make sure it is completely even. Bake for 15-18 minutes until set and golden brown, then take out to cool.
Reduce oven temperature to 150°C/300°F.
Make Lemon Curd
In a small saucepan, add the lemon juice, cornstarch, lemon zest, salt and half of your sugar. Cook over medium heat while whisking until the sugar has dissolved. Once it begins to boil, whisk constantly until it has thickened slightly and then remove from the heat. This part in total takes about 5 minutes.
In a medium bowl, whisk the egg yolks, egg and rest of the sugar until it is smooth and thick. This will take about 1 minute.
While whisking, slowly pour in your hot lemon mixture into the eggs until you have added half of the lemon mixture.
Pour the egg mixture into the saucepan and whisk constantly over medium-low heat, until it is thick enough to cover the back of a spoon and running a finger through it it should leave a line - this will take up to 5 minutes.
Remove from the heat and whisk in your butter, a small piece at a time, making sure you allow the butter to completely melt before adding the next piece.
Pour the still hot curd over your crust, use an offset spatula to cover it evenly. Bake for 30-35 minutes, the middle will still be a bit jiggly but the sides will have set. Remember to lower the oven temperature!
Let the lemon bars cool completely at room temperature, then store in the fridge for at least 3 hours before slicing. Slice and then sift powdered sugar on top.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘US Customary’ above ingredients. I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients about an hour or two before you start baking. Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Graham Crackers: If you don't have graham crackers you can substitute in digestive biscuits.Cornstarch: If you're based in the UK, you can use cornflour. Storage: Keep in an airtight container in the fridge for up to 7 days.