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Heat the oven to 375°F. Coat a 9x13 baking dish with nonstick spray.
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Peel the sweet potatoes and cut them into rounds about ½ inch thick. Spread them in the baking dish and season with the salt.
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Add the butter, maple syrup, brown sugar, cinnamon, nutmeg and ginger to a medium saucepan. Cook over medium-high heat, stirring constantly, until everything melts and reaches a gentle boil.
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Let the glaze simmer for 2 minutes. Remove the pan from the heat and stir in the vanilla.
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Pour the glaze over the sweet potatoes and stir to coat every slice.
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Bake for 60 minutes, stirring every 20 minutes.
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Garnish with fresh rosemary, if using, and serve warm.