Adapted from Anne Quatrano's Summerland: Recipes for Celebrating With Southern Hospitality (Rizzoli).
Sweet potatoes: Preheat oven to 350 degrees. Prick each potato in several places with the tines of a fork. Bake until very soft, 1 to 1 1/2 hours, depending on size. Let stand until cool enough to handle.
Butter a 9-by-13-inch baking dish. Halve potatoes, scoop out flesh, and transfer to a food processor; puree until smooth. Add butter, syrup, ginger, cinnamon, nutmeg, white pepper, and salt; pulse to combine. Transfer to prepared dish, cover, and bake until hot, about 10 minutes.
Meringue: Meanwhile, beat egg whites, cream of tartar, and salt on low speed until frothy, about 30 seconds. Increase speed to high; beat until soft peaks form, about 1 minute. Gradually add sugar, beating until thick, glossy peaks form, about 5 minutes. Spoon meringue evenly over potatoes. Holding a small kitchen torch at a 90-degree angle, 3 inches from surface of meringue, move flame back and forth over meringue until it starts to brown. (Or broil 6 to 8 inches from heat source, about 1 minute.) Serve immediately.