Roll out pie dough on a lightly floured surface into an 11-inch circle. Fit into a 9-inch pie pan and trim edges so they overhang by 1 inch. Fold dough under to create a double-thick edge; crimp decoratively. Place in refrigerator to chill while preparing filling.
Preheat oven to 350ºF. Set a rack in lower third of oven. Line a baking sheet with foil and set it on oven rack. In a small bowl, stir together cinnamon and 1 Tbsp. sugar. In a large bowl, whisk together melted butter, buttermilk, egg yolks, flour, vanilla, salt and remaining 1 cup sugar. Pour into chilled pie shell. Sprinkle cinnamon-sugar over custard.
Set pie on preheated baking sheet and bake for 20 minutes. Without opening oven, reduce temperature to 325ºF and bake until pie is set, puffed and golden, about 20 minutes longer. Let cool on a wire rack, then serve, or cover loosely with plastic wrap and refrigerate to serve cold.