Optional for garnish: chopped fresh or dried rosemary, flaky sea salt
Directions
Peel then slice the sweet potatoes into 1/2-inch thick slices. Place them in a greased 9×13-inch or other 3-quart baking dish. Sprinkle salt on top and toss to coat.
Preheat oven to 375°F (191°C).
Make the sauce: Cut the stick of butter in half to help it melt easier. 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 to stir the sweet potatoes every 20 minutes. After the first 20 minutes, cover the dish with aluminum foil to bake evenly.
Remove from the oven, sprinkle with rosemary and sea salt (if using), then cool uncovered for 10 minutes before serving. The sauce is thin right out of the oven but thickens as it cools.
Cover and store leftovers in the refrigerator for up to 1 week. The sauce will be thick after refrigeration but thins out as you warm the leftovers up. Simply warm in the microwave.