1. Preheat the oven to 325°. In a food processor, pulse the graham crackers with the sugar and salt until finely ground but not powdery. Add the butter and pulse until the crumbs are evenly moistened; transfer to a 9-inch springform pan and press into the bottom and two-thirds up the side. Set the pan on a rimmed baking sheet.
2. In a medium bowl, whisk the condensed milk with the lemon juice. In another bowl, using a handheld mixer, beat the lemon zest with the egg yolks until pale. Beat in the condensed milk mixture until smooth. Pour the filling into the crust.
3. Bake the pie for 25 minutes, until the center jiggles slightly and the edges are set. Transfer the pan to a rack; let cool for 1 hour. Loosely cover the pan with plastic wrap and freeze the pie for at least 6 hours.
4. Wrap a warm, damp kitchen towel around the side of the springform pan to release the pie; remove the ring. Using a hot knife, slice the pie, transfer to plates and serve.