Heat the oven to 375°F and spray a 9x13 baking dish with nonstick spray.
Peel the sweet potatoes and slice them ½ inch thick.
Spread them in the dish and sprinkle with salt.
In a medium saucepan, combine the butter, maple syrup, brown sugar, cinnamon, nutmeg and ginger.
Cook over medium-high heat, stirring constantly, until melted and gently boiling. Simmer for 2 minutes.
Remove from the heat and stir in the vanilla.
Pour the syrup over the sweet potatoes and stir to coat.
Bake for 60 minutes, stirring every 20 minutes, until tender and glossy.
Garnish with fresh rosemary if desired.