Preheat oven to 400 degrees F.
Poke sweet potatoes with a fork all over and roast in oven on a baking sheet for 45 minutes-1 hour. Let cool until cool enough to touch, then run through a potato ricer, a food mill, or a high powered blender/food processor to remove strings and make smooth. Put into a large bowl and set aside.
In a small bowl, sprinkle gelatin over ¼ cup of water and let stand until bloomed, about 3 minutes.
Meanwhile, heat the half and half in a small saucepan until it starts to bubble around the edges. Turn heat off and cover the pan with a lid.
In a medium heatproof bowl (I suggest a pyrex bowl), beat the egg yolks with a whisk until they thicken slightly, for about a minute or two. Add ¼ cup of the granulated sugar and beat until very thick, about 3 minutes. Gradually beat in the warmed half-and-half until well combined.
Fill the now-empty saucepan with 1 inch of water and bring to a simmer. Set the bowl with the egg yolk-half and half mixture over the simmering water and whisk constantly until thickened, about 6-8 minutes.
Add the gelatin to your thickened custard and cook for 1 minute, whisking well to dissolve the gelatin. Remove the bowl from the pan.
Stir the custard into the bowl of sweet potatoes. Add 1 tablespoon of vanilla, the spices and the salt and mix until well incorporated. Set aside.
In a clean sauce pan, pour in ½ cup water and 1 cup granulated sugar. Heat over high heat, and using a candy thermometer, let boil until the syrup is 240 degrees F.
While the syrup is boiling, place egg whites and cream of tartar in the large bowl of a stand mixer with the whisk attachment. Beat the egg whites on medium speed until they are fluffy and have soft peaks (when the whisk is lifted the peaks come up and fall gently back onto themselves. The whites will still look a little foamy, not glossy.)
When syrup is at 240 degrees F, while mixer is moving at high speed, pour the hot syrup in. Keep the mixer on high speed until the meringue is glossy, white, and has stiff peaks.
Using a large rubber spatula, gently fold the egg whites into the sweet potato-custard mixture until no streaks remain.
Pour the filling into the pie crust and refrigerate the pie until very cold and firm, at least 8 hours.
Decorate with whipped cream or serve without.