• Preheat oven to 350°F (180°C).
• Add the graham crackers and sugar to a food processor and blend into fine crumbs. Blend in melted butter to create a thick and crumbly mixture.
• Press the mixture evenly but not too hard into a 9-inch pie pan. Use the back of a spoon or measuring cup to smooth it out for a clean look.
•  Bake the crust for 15 minutes, remove from the oven and cool.

The filling

• Add the egg yolks to a medium bowl and whisk them together by hand. Set aside.
• Whisk water, sugar, lemon juice, cornstarch, salt, and lemon zest in a medium saucepan over medium heat. Let it boil, whisking regularly until it becomes thick and glossy.
• Take it off the heat, and add a tablespoon at a time of the hot mixture into the egg mixture, whisking while you do. Once you've added about 5 tablespoons worth and the egg yolks have warmed, add everything back into the saucepan and place it over low-medium heat. Cook until the mixture is thick and boiling again, whisking continuously. 
• Remove it from the heat and stir through the butter. Dollop this mixture into the chilled crust. If you want extra smooth curd, pass it through a fine-mesh strainer first. 
• Let the curd cool to room temperature, then cover it with a piece of plastic wrap and let it chill in the refrigerator for at least 4 hours until cold and set. 

Swiss meringue

• Combine egg whites and sugar in the bowl of your metal stand mixer or a heat-proof mixing bowl. Place the bowl over a pot of simmering water, ensuring the bottom of the bowl doesn’t touch the water.
• Whisk the egg white mixture until the sugar has dissolved when you rub some between your fingertips. If you have a food thermometer, it should read 160°F/71°C.
• Transfer the warm egg white mixture to a stand mixer fitted with a whisk attachment. Add in the vanilla paste and a pinch of salt. Start on a low speed, gradually increasing to high speed. Whisk until stiff peaks form, and the mixture has cooled and is very thick and glossy. 

Assembling and serving

• Dollop the meringue over the lemon filling and use an offset spatula to spread it in an even layer. You could also use a piping bag to add the meringue. Add in a few glossy peaks and dips to the meringue. Torch the meringue with a blow torch so it's a deep golden brown. Alternatively, place it under a hot broiler for 2-4 minutes until browned on top.
• Slice the pie with a sharp knife. If you want clean slices, wipe the knife in between each cut. Store leftover pie covered in the refrigerator for 2-3 days.