Preheat oven to 325
With an electric mixer, beat egg yolks slightly
Mix flour and sugar together, add to egg yolks
Beat until creamy and pale yellow
Add melted butter, mix well
Add buttermilk, vanilla, lemon juice, lemon zest
In a separate bowl, beat egg whites to soft peaks.
Fold egg whites into pie mixture
Pour into pie shell
Bake for 45 to 60 minutes