Preheat oven to 375°F. Mix 1/4 cup of the brown sugar, orange juice, vanilla and 1 teaspoon of the pumpkin pie spice in large bowl. Add sweet potatoes; toss to coat. Spoon into 13x9-inch baking dish. Dot with 2 tablespoons of the butter. Cover.
Bake 45 minutes or until potatoes are slightly tender.
Meanwhile, mix flour, remaining 1/4 cup brown sugar and remaining 1/2 teaspoon pumpkin pie spice in medium bowl. Cut in remaining 2 tablespoons butter with fork until crumbly. Stir in pecans and marshmallows.
Remove casserole from oven and stir gently to coat potatoes in syrup. Sprinkle evenly with marshmallow-crumb topping. Bake, uncovered, 7 to 10 minutes longer or until potatoes are tender and topping is golden brown.