Preheat your oven to 350°F (175°C). Butter a 9x13 inch baking dish.
In a medium saucepan, combine the granulated sugar, butter, and water. Cook over medium heat, stirring occasionally, until the butter is melted and the sugar is dissolved. Bring to a gentle simmer and cook for about 5 minutes, allowing the syrup to slightly thicken. Stir in the cinnamon, nutmeg, ginger (if using), and salt.
Arrange the sliced sweet potatoes in a single layer in the prepared baking dish. Pour the warm syrup evenly over the sweet potatoes.
Cover the baking dish tightly with aluminum foil. Bake for 45-60 minutes, or until the sweet potatoes are tender when pierced with a fork. Cooking time may vary depending on the thickness of your slices.
Remove the foil. Carefully arrange the mini marshmallows over the top of the sweet potatoes. Return the dish to the oven, uncovered, for another 5-10 minutes, or until the marshmallows are puffed, golden, and gooey. Watch closely to prevent burning.
Let the candied yams rest for a few minutes before serving. This allows the syrup to set slightly and the marshmallows to cool just enough to handle.