Heat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving enough overhang on two sides to lift out the chilled bars.

Stir the graham-cracker crumbs, melted unsalted butter, granulated sugar, and salt together until every crumb is evenly moistened. Press about two-thirds of the mixture firmly into the parchment-lined pan and reserve the rest for the topping.

Whisk the sweetened condensed milk, egg yolks, fresh lemon juice, and finely grated lemon zest in a bowl until smooth and slightly thickened. Pour the lemon mixture over the crust and spread it into an even layer.

Sprinkle the reserved graham mixture over the lemon filling, covering the surface as evenly as possible. Bake for 25–30 minutes, until the topping is lightly golden and the center is just set with a slight jiggle.

Cool the pan completely on a rack, then refrigerate the bars for at least 2 hours. Use the parchment overhang to lift the slab out, wipe a sharp knife between cuts, and slice into 16 squares.