preheat oven to 400

thaw crust 10 minutes

bake til golden - 12 minutes - cool

reduce heat to350

blend brown sugar and flour in food processor

add eggs, yolk, and vanilla and process til blended

add buttermilk and melted butter - process just to blend

pour into cooled pie shell

bake til filling puffs and is almost set, it center still moves slightly when shaken - about 45 minutes

cool, then chill at least 2 hours and up to 1day