Preheat an oven to 180c (350f) and prepare a 9 inch pie pan on the side.
Make the crust: Pulse graham crackers in a food processor until fine crumbs form. If you don’t have a food processor on hand, place the crackers in a Ziplock bag and smash the crackers with a cup until fine crumbs form.
Into a small to medium mixing bowl, add the graham crackers crumbs, and melted butter. Mix with a spoon until the crumbs are coated with butter.
Transfer the crumbs into a prepared 9-inch pie pan, pressing evenly and firmly into the pan. Bake in a 180c (350f) preheated oven for 8 minutes. Remove from the oven and let it cool while you make the filling.
Into a large mixing bowl, add the sweetened condensed milk, egg yolks, lemon juice, lemon zest, and vanilla extract (optional), and mix with a whisk until well combined.
Pour the pie filling onto the graham cracker crust and bake in a 180c (350f) preheated oven for 25 minutes or until the edges of the pie are set and the middle is still wobbly when you shake it.
Allow the pie to cool to room temperature for at least an hour, and let it set in the fridge for at least 4 hours. Overnight is best. Top the pie with freshly made whipped cream, and lemon zest.
