2 medium sweet potatoes
4 tbsp. unsalted butter, melted
2 tbsp. fresh lemon juice
1/2 tsp. grated nutmeg
1/2 tsp. ground cinnamon
1/2 tsp. kosher salt
3 large eggs, separated
1/2 cup sugar
2 tbsp. flour
3/4 cup buttermilk
1 pie crust* (see recipe at bottom)
Preheat oven to 375°F. 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.
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.
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. Remove pie from oven and cool completely on a rack. Serve at room temperature with a scoop of ice cream on the side.
PIE CRUST RECIPE
1 1/2 cup sifted, flour
1 tbsp. sugar
1 tsp. salt
1 stick cold unsalted butter, cut into small pieces
1/4 cup ice water
Sift dry ingredients together in a medium bowl. Sprinkle in butter and refrigerate 30 minutes. Dust work surface with flour. Preheat oven to 325°F. Cut butter into dry ingredients using a pastry blender or your fingertips until mixture resembles coarse crumbs. Add ice water, a tablespoon at a time and tossing mixture with a fork to combine after each addition, until pastry holds together when pinched. Gather pastry into a round disk, wrap in plastic wrap, and refrigerate for 15 minutes or until ready to use. Roll out dough to a 12-inch round. Transfer to a 9-inch pie pan. Fold any excess dough that hangs below pan rim on top of rim so you have enough to crimp. Cut off excess to patch any holes or tears. Refrigerate for 15 minutes before filling or prebaking. Lay a sheet of aluminum foil over dough and carefully scatter pie weights in pan. Bake on middle rack for 12 to 15 minutes. Remove pie weights and foil, prick bottom of crust with a fork, and bake until bottom of crust appears dry, about 10 minutes.