Peel the sweet potatoes and cut them into 1-inch chunks, keeping pieces uniform for even cooking.
Place the sweet potato chunks in a large pot and cover with cold water. Bring to a boil over medium-high heat, then reduce to a simmer. Cook for 5-7 minutes until potatoes are just beginning to soften but still hold their shape.
While the potatoes are par-boiling, combine brown sugar, butter, maple syrup, cinnamon, nutmeg, salt, vanilla extract, and orange juice in a small saucepan. Heat over medium-low until the butter melts and ingredients are thoroughly combined, about 3-4 minutes.
Preheat your oven to 375°F (190°C). Drain the sweet potatoes thoroughly and transfer them to a 9x13 inch baking dish.
Pour the warm glaze evenly over the potatoes, gently tossing to ensure each piece is coated. For even more flavor infusion, let the mixture stand for 5 minutes before baking.
Bake uncovered for 40-45 minutes, basting the potatoes with the glaze from the bottom of the dish every 15 minutes. The potatoes are done when they're tender all the way through and the glaze has reduced to a thick, shimmering syrup.
If using marshmallows, sprinkle them over the sweet potatoes during the last 5 minutes of baking. Watch carefully to ensure they turn golden brown without burning.