Peel then slice the sweet potatoes into ½ inch thick slices. Place in a greased baking dish. Sprinkle salt on top and toss to coat.

Preheat oven to 375°F (191°C).

Cut stick of butter in half. Combine all the butter, water, maple syrup, brown sugar, cinnamon, nutmeg, and ginger into a medium saucepan over medium heat. Cook and stir until the butter has melted. Stop stirring and bring to a gentle boil. Boil for 2 minutes without stirring. Remove sauce from heat and stir in the vanilla extract. (And orange zest, if using.)

Pour sauce over potatoes and toss to coat.

Bake for 1 hour, stopping and stirring the sweet potatoes every 20 minutes. After the first 20 minutes, cover the dish with aluminum foil.

Remove from the oven, sprinkle with rosemary and sea salt (if using), then cool uncovered for 10 minutes before serving.

Cover and store leftovers in the refrigerator for up to 1 week.