Preheat the oven to 400°F. Place the whole sweet potatoes on a parchment-lined baking sheet.
Bake for approximately 40 minutes, until partially softened but still firm enough to slice.
Allow the potatoes to cool until safe to handle. Peel and cut into approximately 1/4-inch slices.
Arrange half of the sweet potato slices in a 12-inch cast-iron skillet.
Sprinkle with half of the granulated sugar, brown sugar, cinnamon, and nutmeg. Distribute half of the butter over the potatoes.
Add the remaining sweet potatoes in a second layer.
Top evenly with the remaining granulated sugar, brown sugar, cinnamon, nutmeg, salt, and butter.
Pour the vanilla extract and water over the potatoes.
Cover the skillet and cook over medium-high heat until the butter and sugars melt and begin forming a bubbling syrup.
Reduce the heat to LOW.
Carefully spoon some of the syrup from the bottom of the skillet over the top layer of sweet potatoes.
Cover and continue cooking over low heat until the potatoes are completely tender, occasionally basting the top with additional syrup.
Once the sweet potatoes are fork-tender, remove them from the heat and allow them to rest briefly.
Serve warm with the candied syrup spooned over the top.