Preheat the oven to 400 degrees Fahrenheit (200 degrees Celsius). Lightly grease a 9x13-inch baking dish.
Peel the yams, cut them into 1-inch pieces about ¼-inch thick, and place them in a large mixing bowl.
In a small bowl, whisk together the brown sugar, melted vegan butter, vanilla, cinnamon, nutmeg, salt, and ginger (if using). Pour it over the yams and toss until they are evenly coated.
Layer the yams in the baking dish, cover with aluminum foil, and bake for 20 minutes. Stir the yams and bake for another 20 minutes, until tender.
Remove the foil, add a layer of vegan marshmallows on top, and bake for 5 to 6 minutes, just until the marshmallows are warm and soft. Don't let them bubble, or they will melt into white goo. For toasted marshmallows, broil for 1 to 2 minutes, watching closely so they don't burn.
Video
Notes
Slice the sweet potatoes evenly so they cook at the same rate. A mandoline or food processor slicing disk makes this quicker.
For more syrupy candied yams, save some of the sauce to baste the yams when you stir them halfway through, or double the sauce. If doubling the sauce, use fewer marshmallows so it isn't too sweet.
Sweeten to taste with more or less brown sugar.
Make ahead by preparing everything the night before. Keep it covered in the fridge, then bake right before serving.
Leftovers keep in the fridge for 4 to 5 days. I haven't tested freezing them.