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Preheat oven to 400 degrees.
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Bake potatoes until just softened, about 1 hour.
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Cool 15 minutes.
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Cut a half inch off of tops of potatoes and scoop out pulp leaving shells intact.
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Mash pulp. Set aside 4 shells and toss out the other two.
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In a small skillet, melt butter.
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Add onion, cook until softened about 8 minutes.
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Add brown sugar, zest, salt, cumin, cinnamon, pepper, and allspice.
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Cook until sugar melts about 2-3 minutes.
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Combine onion mixture with mashed sweet potatoes.
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Stir in sour cream and parsely.
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In a seperate bowl, mix egg whites at high speed until soft peaks form.
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Gently fold into sweet potato mixture.
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Spoon back into Potato shells.
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Put on baking sheet and bake until filling is just set and tops begin to brown about 15-20 minutes.