1 Preheat the oven to 375° F.
Lightly grease a 1 1/2 quart souffle dish or deep baking dish; dust with 2 teaspoons of the sugar.
In a food processoror blender, whirl the potatoes, 2 tablespoons of the remaining sugar, the milk, cinnamon, allspice, 1/4 teaspoon of the salt, the ground ginger, and nutmeg for 45 seconds or until smooth.
Trans-ferto a large bowl and stir in the crystallized ginger.
2 In a perfectly clean medium-size bowl, beat the egg whites with an electric mixer set on high until foamy.
Add the remaining 1/4 teaspoon of salt and the cream of tartarand beat for 1 minute or until very soft peaks form.
Gradually add the remaining 2 tablespoons of sugar, beating continuously for 3 more minutes or until the whites are stiff and glossy but not dry.
Stir 1/4 of the egg whites into the sweet potato mixture to lighten it, then gently fold in the remaining egg whites.
Scrape the mixture into the prepared dish and bake for 45 minutes or until puffed and golden brown.