MAKE THE CRUST: Preheat the oven to 350°F. In the bowl of a food processor, pulse the shortbread cookies until the crumbs are the consistency of coarse sand (you should have about a cup and a half of crumbs). Transfer the crumbs to a medium bowl, add the sugar and melted butter, and stir to combine.
Pour the crust mixture into a 9-inch springform pan or pie dish and press evenly over the bottom of the pan. Bake until just golden and puffed, 8 to 10 minutes. Set aside to cool completely before adding the filling.
MAKE THE FILLING: In the bowl of a stand mixer fitted with the whisk attachment, combine the egg yolks, sugar, and salt and whip on high speed until the mixture is light in color, 3 to 4 minutes. With the mixer running, slowly add the sweetened condensed milk, and then the lemon zest and juice. Pour into the prepared pan and freeze for at least 6 hours, or up to overnight.
When ready to serve, remove from freezer, un-mold the pie, slice into wedges and serve with freshly whipped cream.
