In a medium bowl, combine the graham cracker crumbs, sugar and salt. Stir in the butter. Transfer to a 9-inch tart pan. Evenly press across the bottom and up the sides to make a crust (I use a metal measuring cup to press the crumbs). Bake for 12 minutes. Let cool completely.
Filling
In a large bowl, combine the condensed milk, lemon zest and juice, sugar, and egg yolks. Whisk until all the ingredients are completely blended. Pour the filling into the cooled crust. Freeze for at least 2 hours.