Preheat the oven to 350°. In a large bowl, using an electric mixer, beat the butter with the sugar at medium speed until light and fluffy. Beat in the egg yolks and flour, then beat in the buttermilk, lemon juice and nutmeg.
In a medium stainless steel bowl, using clean beaters, beat the egg whites until stiff but not dry. Fold the beaten whites into the buttermilk mixture until no white streaks remain. Pour the custard into the baked Flaky Pie Shell and bake in the center of the oven for 45 minutes, until the custard is golden and risen, or a toothpick inserted in the center comes out clean. Let cool and serve.
The pie can be refrigerated overnight and served at room temperature or lightly chilled.