Step 1

Crust: 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.

Step 2

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.

Step 3

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.

Step 4

Filling: 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.