Add butter; pulse until pea-size pieces of butter form.
Add 1/2 cup buttermilk; pulse until moist clumps form, adding more buttermilk by tablespoonfuls if too dry. Form into a ball; flatten into a disk. Wrap in plastic and chill for 1 hour.
Preheat oven to 350°F.
Roll out dough on a lightly floured surface to a 14" round.
Transfer to 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°F.
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 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.