- 1½ pounds sweet potatoes (about 2 medium potatoes), peeled and chopped into ½-inch dice
- 4 tablespoons unsalted butter, melted
- 2 tablespoons fresh lemon juice
- ½ teaspoon freshly grated nutmeg
- ½ teaspoon ground cinnamon
- ½ teaspoon kosher salt
- 3 large eggs, separated
- ½ cup sugar
- 2 tablespoons all-purpose flour
- ¾ cup whole or lowfat buttermilk (preferably whole)
- 1 Sweet Pie Crust , prebaked
- Preheat the oven to 375 degrees.
- Pour 1½ inches of water into a 3-quart stockpot with a strainer basket and bring to a boil over medium-high heat. Add the sweet potatoes, cover, and steam until fork-tender, about 20 minutes. Strain the 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 1¼ cups puree; discard any excess. Add the butter, lemon juice, nutmeg, cinnamon, and salt, mixing thoroughly with a wooden spoon or rubber spatula after each addition.
- In a small bowl, beat the egg yolks lightly with a whisk, about 30 seconds. Add the sugar and beat until they’re a creamy lemon-yellow color, about 1½ minutes. Add the egg mixture to the sweet potato mixture and stir with a wooden spoon or rubber spatula until the eggs are thoroughly incorporated and the filling is a consistent bright orange color. Add the flour a little at a time, stirring after each addition, until thoroughly incorporated. Add the buttermilk and stir to incorporate.
- Wash the whisk in a stream of hot water to wash away any butter residue, then rinse in cold water to cool it down and dry with a paper towel. In a separate bowl, whisk the egg whites to soft peaks, about 1½ minutes. With a wooden spoon or rubber spatula, gently fold the egg whites into the sweet potato-buttermilk mixture until thoroughly combined. Pour the mixture into the prepared pie crust and bake on the middle rack until the center is firm and set, 35 to 40 minutes.
- Remove the pie from the oven and cool completely on a rack. Serve at room temperature (or cover with plastic wrap, chill in the refrigerator, and serve cold), with a dollop of whipped cream and a mint leaf on top.