- Preheat oven to 350 degrees. In the bowl of a stand mixer fitted with whisk attachment, combine butter and sugar until sugar is completely incorporated. Add egg yolks and mix well to combine. Add flour, lemon juice, nutmeg, and salt. With the mixer running, slowly add buttermilk. Mix well and set aside.
- In another bowl, whip egg whites until they form soft peaks. Pour a small amount of buttermilk mixture into whites. Fold gently to combine. Gently fold egg white mixture into remaining buttermilk mixture until just combined. Pour custard into baked pie shell.
- Bake in middle of oven until filling is lightly browned and barely moves when the pie is jiggled, about 45 to 50 minutes. Cool on a rack and serve warm or at room temperature. Any leftovers should be refrigerated.
For the pie crust:
- In the bowl of a food processor, combine all ingredients except ice water and pulse to combine. Transfer to a large mixing bowl and add ice water, mixing to combine.
- On a floured work surface, form dough into a disc. Wrap in plastic and refrigerate for at least 30 minutes.
- Preheat oven to 425 degrees. Remove dough from refrigerator and allow to sit at room temperature 5 to 10 minutes. Using a rolling pin and starting and the center and working outward, roll dough into a large circle.
- Transfer to a pie pan and gently press dough into pan so it lines the bottom and sides. Use a butter knife to trim excess dough, then crimp edges to pie dish.
- Place a large piece of parchment paper on top of dough, then fill with dried beans or pie weights. Bake for 10 minutes. Remove parchment and beans and prick dough all over with a fork, then continue baking until crust is golden and dry, about 10 more minutes. Let cool completely on a rack before filling.