2 cups all-purpose flour plus more
1 teaspoon kosher salt
1 teaspoon sugar
3/4 cup (1 1/2 sticks) chilled unsalted butter cut into 1/2” cubes
1/2 cup (or more) cold buttermilk
1 1/2 cups sugar
1/2 cup (packed) light brown sugar
1 1/2 tablespoons yellow cornmeal
1 tablespoon all-purpose flour
5 large eggs, beaten to blend
2/3 cup buttermilk
1/2 cup (1 stick) unsalted butter, melted
1 3/4 tablespoons fresh lemon juice
1 tablespoon freshly grated lemon zest
2 teaspoons vanilla extract
Pinch of kosher salt
9 1/2”-diameter deep-dish pie pan
Mix flour, salt, and sugar in a food processor. Add butter; pulse until pea-size pieces of butter form. Add 1/2 cup buttermilk; pulse until moist clumps form, adding more buttermilk by tablespoons if too dry. Form into a ball; flat into a disk. Wrap in plastic and chill for 1 hour.
Preheat oven to 350°. Roll out dough on a lightly floured surface to a 14” round. Transfer to a pie pan; gently press onto bottom and up sides of pan. Trim dough leaving a 1” overhang; tuck overhang under. Crimp edges decoratively. Line with parchment paper or foil; fill with pie weights or dried beans.
Bake crust until edges begin to brown, 30–35 minutes. Remove paper and weights; bake until golden brown, 25–30 minutes longer. Let cool completely.
Preheat oven to 350°. Whisk first 4 ingredients in a medium bowl until well combined. Whisk eggs and remaining 6 ingredients in a large bowl (mixture may look curdled). Slowly whisk in dry ingredients. Pour filling into a cooled crust and bake until custard is set around edges but jiggles slightly in center, 1 hour–1 hour 15 minutes. Let cool completely on a wire rack.DO AHEAD: Can be made 1 day ahead. Cover and refrigerate. Bring to room temperature before serving.