Add the crackers and sugar to a food processor and pulse a few times. Add the melted butter and lemon zest, then pulse until the crumbs look like wet sand, about 30 seconds. Press crumb mixture evenly into the bottom and up the sides of a 8-inch pie pan, then chill for at least 15 minutes. Bake the crust until the edges start to brown, 15 to 17 minutes. Let cool slightly.
For the filling: Add the condensed milk and egg yolks to a medium bowl and whisk until the yolks are incorporated. Add the lemon juice and whisk until fully combined, another 1 to 2 minutes. Pour into your prepared pie shell and bake until the filling has nearly set in the center, 20 to 25 minutes. Cool to room temperature, then chill in the refrigerator for at least 2 hours.
Top with whipped cream and freshly grated lemon zest and serve.