Preheat the oven to 375ºF (190ºC) and line an 8-inch (20-cm) square baking dish with parchment paper. Leave some paper hanging over the sides of the pan to use as handles to lift the slices out once baked.
In a mixing bowl, stir together the graham cracker crumbs, sugar, salt, and melted butter until well combined. Dump the mixture into the prepared square pan and press the crumbs into an even layer with your fingertips. Place the graham crust in the preheated oven and bake it for 6 to 8 minutes. Remove the crust from the oven, and set it aside on a wire cooling rack.
Reduce the oven temperature to 350ºF (180ºC).
Whisk the condensed milk, lemon juice, egg yolks, and lemon zest together in a mixing bowl until smooth. Pour the lemon filling over the pre-baked graham crust. Place the pan back in the oven and bake the slices for 13 to 15 minutes, or until the filling is slightly jiggly but not wet. Transfer the pan to a wire rack to cool completely before slicing.
When the lemon slices have cooled completely, use the parchment paper handles on the sides of the pan to gently wiggle and coax them from the bottom of the pan. It may take a few tries for the graham crust to release. Once released, use the parchment paper handles to gently lift the lemon slices to a cutting board. Using a long sharp knife, carefully cut the whole thing into four large squares, then cut each square into three individual condensed milk lemon slices and serve.
Notes
Because the lemon filling is so creamy, and the graham crumb bottoms are delicate, we prefer eating condensed milk lemon slices with forks rather than as a handheld sweet.You must be careful when cutting the slices since the graham crumb bottom is fairly crisp. Using too much force puts your hands at risk of slipping and squishing the creamy lemon filling if the knife cuts through suddenly.Condensed milk lemon slices are best eaten the day they're made. They can be stored in an airtight container in the fridge for two days, however, they may begin to weep. A good alternative is to gently place them in an airtight container in the freezer for up to one week. Simply remove however many you need from the freezer, and let them thaw at room temperature for 30 minutes before serving.
SUBSTITUTIONS
Chocolate crumbs are a great substitute for graham cracker crumbs if you like. That being said, any crumb bottom you think would be good with lemon is fine. Just substitute the crumbs at a 1:1 ratio.You can substitute any citrus for lemon if you like. Grapefruit, lime, and orange are all delicious. I've made them with pink grapefruit and added a drop or two of red food colouring to the filling to give it a light pink hue.I once made condensed milk lemon slices with the pulp of one passionfruit added to the lemon filling and it was incredibly delicious!