Peel then slice the sweet potatoes into 1/2 inch thick slices. Place 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 stick of cannabutter in half. Combine all the cannabutter, 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.
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.