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Preheat to 450°F Place rack in middle.
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Separate eggs, putting yolks in a large bowl and whites in a medium bowl.
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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.
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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.
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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.
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Cool pie completely to room temperature on a rack before serving.