Preheat your oven to 400ºF/200ºC.
Peel the sweet potatoes, then use a sharp knife to cut them into thick rounds, about 1/2 inch thick. Overlap the sliced sweet potatoes in a 9x13 baking dish. Set aside.
In a small saucepan, combine the sauce ingredients: sugar, potato starch or cornstarch, kosher salt, and orange juice. Whisk to combine then bring to a gentle boil over medium heat. Cook, stirring often until the sauce thickens, about 1 minute.
Remove from the heat, add the vegan butter and orange zest. Stir until the butter has melted then pour over the top of the yams/sweet potatoes.
Bake, covered with an oven-safe lid or aluminum foil for an hour to an hour and a half. Uncover and baste the sweet potatoes with the sauce a couple of times throughout the cooking process. They're done when the sweet potatoes can easily be pierced with a fork.