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1
Preheat the oven to 375F.
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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 let steam until fork tender, about 20 minutes.
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4
Strain the the sweet potatos, 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 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 well 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 they're 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 spatula until the eggs are fully 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 incorporated.
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13
In a separate bowl, whisk the egg whites to soft peaks, about 1-1/2 minutes.
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14
Gently fold the egg whites into the sweet potato mixture until thoroughly combined.
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15
Pour the mixture into the prepared pie shell and bake on the middle rack until firm and set,about 35-40 minutes.
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16
Cool on a wire rack.
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17
Serve with a dollop of whipped cream.
