Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
Peel the sweet potatoes and slice them into ½-inch (1.25 cm) thick rounds.
Arrange the slices evenly in the prepared baking dish.
In a saucepan over medium heat, melt the vegan butter. Once melted, stir in the milk and heat until warmed through.
Mix the sugar, ground cinnamon, nutmeg, cloves, cardamom, and salt into the saucepan, and whisk together until the sugar dissolves.
Pour the sweet syrup evenly over the sweet potatoes, ensuring the slices are well-coated. Cover the dish tightly with foil.
Bake for 30 minutes at 350°F (175°C).
After 30 minutes, remove the foil, gently stir the sweet potatoes.
Continue baking, uncovered, for an additional 25-30 minutes, until the sweet potatoes are a little more than fork-tender.
If you are using them, add marshmallows to the top and put the tray under the broiler for about a minute to lightly brown the marshmallows. If your oven doesn’t have a broiler, add them just 15 minutes after removing the foil, giving them 15 minutes to melt and brown before removing the pan from the oven.