Preheat oven to 300ºF
Prepare your favorite un-bake pie crust in a 10" pie tin
Use mixer with paddle attachment, at medium high, whip butter & sugar until fluffy, pale yellow
Add egg yolks, one at a time, mix well, add vanilla
Turn speed down to 2, slowly add buttermilk & flour to the mixture; put mixture aside
In a clean oil-free mixing bowl, beat egg whites to foamy, then add sugar; beat to soft-peak — or it would not mix well with the rest
Gently fold the whipped egg white into the egg yolk mixture until well blended, clumps are ok
Pour mixture into the prepared pie crust; baked at pre-heated oven (300ºF) for about 1hr (check by insert a skewer, if it pulls out clean, it's done!)
Keep in refrigerator overnight, or at least 8 hrs; served chilled, with your favorite cup of coffee for breakfast, why not?