Preheat to 450°F Place rack in middle.

Separate eggs, putting yolks in a large bowl and whites in a medium bowl.

Briefly whisk the yolks, then whisk in 1/3 cup of the sugar, the flour and the salt. Stir in the butter, and then the buttermilk and vanilla. Whisk until completely combined.

Beat egg whites with electric mixer until soft peaks form. Continue beating, ad remaining 1/3 cup sugar, 1 TBSP at a time, until whites are stiff and glossy. Stir 1 cup of the whites into the yolk mixture; blend thoroughly. Spoon the lightened yolk mixture onto the remaining whites in the large bowl. Using a whisk, fold together gently but thoroughly. Pour the filling into the prepared pie shell.

Bake for 10 minutes. Reduce temperature to 350°F Bake for another 20-25 minutes. The top of the pie should be dappled with brown and tenderly evenly set.

Cool pie completely to room temperature on a rack before serving.