Preparation Position rack in center of oven and preheat to 400°F. Thaw pie crust 10 minutes. Bake crust until pale golden, about 12 minutes. Transfer to rack and cool. Reduce oven temperature to 350°F. Blend brown sugar and flour in processor. Add eggs, yolk and vanilla and process until blended. Add buttermilk and melted butter and process just to blend. Pour mixture into cooled crust. Bake pie until filling puffs and is almost set but center still moves slightly when pan is shaken, about 45 minutes. Cool pie on rack to room temperature. Chill until cold, at least 2 hours and up o 1 day. Cut into wedges and serve.
Preparation Position rack in center of oven and preheat to 400°F. Thaw pie crust 10 minutes. Bake crust until pale golden, about 12 minutes. Transfer to rack and cool. Reduce oven temperature to 350°F. Blend brown sugar and flour in processor. Add eggs, yolk and vanilla and process until blended. Add buttermilk and melted butter and process just to blend. Pour mixture into cooled crust. Bake pie until filling puffs and is almost set but center still moves slightly when pan is shaken, about 45 minutes. Cool pie on rack to room temperature. Chill until cold, at least 2 hours and up o 1 day. Cut into wedges and serve.