Prepare the Sweet Potatoes : Preheat your oven to 350°F (175°C). Transfer the peeled and cubed sweet potatoes into a large pot and add enough water to cover them. Bring to a boil and cook for 15–20 minutes, or until the potatoes are tender. Drain the sweet potatoes and mash them until smooth.

Make the Sweet Potato Mixture : In a large mixing bowl, combine the mashed sweet potatoes with milk, orange juice, brown sugar, softened butter, beaten egg, cinnamon, vanilla extract, and salt. Mix until well blended and creamy. Transfer the mixture to a greased 9x13-inch baking dish, spreading it out evenly.

Prepare the Topping : In a medium bowl, combine the chopped pecans, softened butter, and dark brown sugar. Mix until the mixture resembles a crumbly topping. Sprinkle the pecan mixture evenly over the sweet potato layer. Scatter the mini marshmallows on top, creating a single layer.

Bake : Bake the casserole in the preheated oven for 25–30 minutes, or until the marshmallows are golden and the topping is crisp. Allow the casserole to cool slightly before serving.