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1
Preheat the oven to 375 degrees.
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2
Pour 1 1/2 inches of water into a 3-quart stockpot with a strainer basket and bring to a boil over medium-high heat.
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3
Add the sweet potatoes, cover, and steam until fork-tender, about 20 minutes.
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4
Strain the sweet potatoes, place in a large bowl, and let cool to room temperature.
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5
Mash them to a smooth puree with a fork or a potato masher.
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6
You should have 1 1/4 cups puree; discard any excess.
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7
Add the butter, lemon juice, nutmeg, cinnamon, and salt, mixing thoroughly with a wooden spoon or rubber spatula after each addition.
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8
In a small bowl, beat the egg yolks lightly with a whisk, about 30 seconds.
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9
Add the sugar and beat until theyre a creamy lemon-yellow color, about 1 1/2 minutes.
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10
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.
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11
Add the flour a little at a time, stirring after each addition, until thoroughly incorporated.
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12
Add the buttermilk and stir to incorporate.
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13
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.
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14
In a separate bowl, whisk the egg whites to soft peaks, about 1 1/2 minutes.
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15
With a wooden spoon or rubber spatula, gently fold the egg whites into the sweet potato-buttermilk mixture until thoroughly combined.
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16
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.
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17
Remove the pie from the oven and cool completely on a rack.
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18
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.
