Preheat your oven to 400°F (200°C). Rinse and pat dry the sweet potatoes, then slice them into even rounds or wedges so they cook uniformly.
Toss the cut sweet potatoes with a little salt and a drizzle of oil if you like, then spread them in a single layer on a baking sheet. Roast for about 20 minutes, turning once, until they’re just tender and caramelized at the edges.
Meanwhile, in a small saucepan over low heat, melt the butter. Whisk in the brown sugar, maple syrup, cinnamon, vanilla, and salt until the mixture is smooth and glossy.
Remove the potatoes from the oven and transfer them to a baking dish. Pour the warm glaze over them, using a spoon to coat each piece evenly. Return to the oven (still at 400°F) and bake for another 10–12 minutes, until the glaze thickens and coats the potatoes with a syrupy sheen.
If you’re adding marshmallows, sprinkle them on top during the last 2–3 minutes of baking, just until they puff and lightly brown. Serve hot or warm to enjoy the full, melty glory.