The night before your meal, clean, peel, and chunk the sweet potatoes. I like to leave mine in bigger-than-bite-sized chunks. Place in a large baking dish. Use a dish big enough that the potatoes are mostly in a single layer.
Combine butter, brown sugar, and orange juice in a mixing bowl. Pour over potatoes. Top with a handful of marshmallows, which will melt into the sauce.
Cover and microwave for 10 minutes, or until the potatoes are tender. If you're making ahead, allow the potatoes to cool and refrigerate until it's time to serve. If not, just move on to the next step.
To serve: Top with remaining marshmallows and bake at 375 for ten minutes (twenty minutes if cold), until the potatoes are hot and marshmallows are just toasted.