In a large bowl, whisk together the sweetened condensed milk and lemon juice until the mixture is smooth, thick, and creamy. It will start to thicken as the lemon juice reacts with the milk—this is what you want.
Pour the lemon cream mixture over the cooled crust, spreading it evenly to the edges with a spatula.
Cover the dish tightly with plastic wrap or foil and refrigerate for at least 4 hours, or overnight if you can, until the filling is fully chilled and set.
When ready to serve, cut into squares. If you like, sprinkle a little finely grated lemon zest on top for extra color and fragrance. Serve cold, straight from the icebox.
For a slightly sweeter, less tart dessert, reduce the lemon juice to 3/4 cup and taste as you go, adding more if you want a brighter flavor. If you prefer a different crust, you can swap the graham crackers for vanilla wafers or shortbread cookies—just crush them the same way and mix with melted butter. A pinch of salt in the crust can deepen the flavor, especially if you’re using unsalted butter. To make this feel a bit more like a special occasion dessert, fold 1 cup of lightly whipped cream into the lemon mixture before pouring it over the crust; it will give you a softer, mousse-like filling. You can also layer the dessert in individual jars or small glasses for parties, pressing a spoonful of crumbs in the bottom and topping with the lemon cream. For a berry twist, scatter fresh blueberries, raspberries, or sliced strawberries over the chilled top just before serving. If you’re cooking for a smaller household, the recipe halves nicely in a loaf pan—slice it like little lemon icebox bars and keep them tucked away for when you want just a taste of something sweet. Always keep leftovers refrigerated, and if the crust softens a bit after a day or two, many folks find they like that almost-cake-like texture even better.