Real Sweet Potato Pie

1.Preheat oven to 375u0b0F. Pour 1 1/2 inches of water into a medium stockpot with a strainer basket and bring to a boil over medium-high heat. Add sweet potatoes, cover, and steam until fork-tender, about 20 minutes. Strain sweet potatoes, place in a large bowl, and let cool to room temperature. Mash them to a smooth puree with a fork or a potato masher (you should have 1 1/4 cups puree; discard any excess). Add butter, lemon juice, nutmeg, cinnamon, and salt, mixing after each addition.
2.In a small bowl, beat egg yolks lightly with a whisk for about 30 seconds. Add sugar and beat until they're a creamy lemon-yellow color, about 1 1/2 minutes. Add egg mixture to sweet-potato mixture and stir until eggs are thoroughly incorporated and filling is a consistent bright-orange color. Add flour a little at a time, stirring after each addition, until thoroughly incorporated. Add buttermilk and stir to incorporate.
3.With a clean, dry whisk and in a separate bowl, whisk egg whites to soft peaks, about 1 1/2 minutes. Gently fold egg whites into sweet potato - buttermilk mixture until thoroughly combined. Pour mixture into prepared piecrust and bake on middle rack of oven until the center is firm and set, 35 to 40 minutes.