Preparation

Bale the pie

• Position a rack in the center of the oven and heat the oven to 400°F.
• In a small bowl, stir together the sugar, cinnamon, ginger, nutmeg, and salt.
• In a large bowl or in the bowl of a stand mixer, beat the evaporated milk, eggs, butter, and vanilla on medium speed until well combined. Add the spice mixture, and beat on low speed to combine well. Add the sweet potatoes and beat, increasing the speed, until the mixture is fairly smooth, 3 to 4 minutes.
• Pour the pie filling into the blind-baked crust, transfer to a rimmed baking sheet, and bake for 10 minutes. (If the edge begins to darken too much, use a pie shield.)
• Reduce the heat to 350°F and bake until the filling puffs up around the edges, loses its shine, and is set with a little jiggle in the center, 50 to 55 minutes. Transfer to a rack and cool to room temperature. If not serving right away, cover the pie with plastic wrap and refrigerate for up to two days.

Make the topping

• Fill a 4-quart saucepan about one-third full of water. Have ready a large stainless-steel mixing bowl that will fit over, but not touching, the water. Bring the water to a boil over high heat, then reduce the heat to maintain a lively simmer.
• Off the heat in the large bowl, begin whisking by hand the egg whites, sugar, corn syrup, cream of tartar, salt, and 1/4 cup water. Switch to a hand-held electric mixer, and beat the ingredients at low speed until foamy and a little less grainy, 2 to 5 minutes. (Check for graininess by rubbing a little between your fingers.) Place the bowl over the simmering water. Beat at medium speed until the ingredients are warm and the sugar completely dissolves (it will feel smooth), about 1 minute. Increase the speed to high and beat, stopping occasionally to scrape the sides of the bowl, until the mixture is thick and shiny and can hold its shape, 4 to 5 minutes.
• Remove from the heat and add the vanilla extract. Beat on high speed until the mixture loses its shine and thickens just a little more, 1 to 2 minutes more.
• Transfer to a medium bowl and let cool to room temperature. For maximum fluffiness, use within an hour or two. (You won’t use all of it on the pie, but it keeps well, covered and refrigerated, for at least a week. Use any leftover as you would jarred fluff.)
• To top the pie, dip two spoons in warm water, shake dry, and use to dollop the topping on the pie. To give the topping some texture, swirl a small spoon over it. Brown the topping briefly with a kitchen torch, if you like, before serving.