Place sweet potatoes in large saucepot and cover with water. Bring to a boil and simmer 20-25 minutes, or until tender; drain.
Place potatoes, milk, butter and pumpkin pie spice in large mixing bowl. Beat with electric mixer until smooth.
Add sweet potato mixture to shallow baking dish, top with pecans and marshmallows. Place under preheated broiler 5 inches from heat; broil 2-3 minutes, or until marshmallows are lightly browned.