Place the sweet potato chunks in a large pot and cover with cold water by 1 inch. Bring to a boil over high heat, then reduce heat to medium and simmer for 20-25 minutes until the potatoes are very tender when pierced with a fork.
Drain the sweet potatoes thoroughly in a colander, then transfer them to a large mixing bowl. Mash with a potato masher or beat with an electric mixer on medium speed until completely smooth with no lumps remaining.
Preheat your oven to 350°F and grease a 9x13-inch baking dish with butter or nonstick cooking spray. Let the mashed sweet potatoes cool for 5 minutes, then stir in the melted butter, granulated sugar, beaten eggs, vanilla extract, salt, whole milk, and orange juice until fully combined.
Spread the sweet potato mixture evenly into the prepared baking dish using a rubber spatula. Bake uncovered for 30-35 minutes until the edges are lightly bubbly and the center is set.
Remove the casserole from the oven and evenly scatter the miniature marshmallows across the entire surface. Return to the oven for 5-7 minutes until the marshmallows are puffed, golden brown, and toasted.
