3/4cup(1 1/2 sticks) chilled unsalted butter cut into 1/2” cubes
1/2cup(or more) cold buttermilk
1/2cup(packed) light brown sugar
1 1/2tablespoonsyellow cornmeal
5large eggs, beaten to blend
1/2cup(1 stick) unsalted butter, melted
1 3/4tablespoonsfresh lemon juice
1tablespoonfreshly grated lemon zest
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.