For the dough: Add the flour and salt to a food processor and pulse to combine. Add the butter and pulse until the butter forms pea-size pieces. Add the water and continue to pulse until a dough begins to come together. Press the dough into a disc and wrap in plastic wrap. Refrigerate until thoroughly chilled, at least 1 hour.
Preheat the oven to 350 degrees F. On a lightly floured surface, roll the dough into a disk about 11 to 12 inches wide. Line the bottom of a 9-inch pie plate with the dough, then crimp the edges. Refrigerate while you make the filling.
For the filling: Whisk together the sugar, buttermilk, eggs, egg yolk, melted butter, lemon juice, lemon zest, vanilla and salt until well blended. Sprinkle in the flour and whisk just until incorporated. Place the pie plate on a baking sheet, then pour the filling over the dough. Bake until the top is golden brown and set around the edges with a slight jiggle in the center, about 1 hour. If the crust begins to get too brown, but the filling isn’t ready, cover the edges loosely with foil. Let the pie completely cool to room temperature before slicing. Serve with whipped cream.