Cover sweet potatoes with cold water and bring to a boil. Reduce to a simmer and cook 12 to 15 minutes or until knife-tender. Drain and return to pot. Mash potatoes with a potato masher, then stir in milk, butter, pineapple, brown sugar, pumpkin pie spice and salt.
Transfer mixture to a 2 1/2-quart oval casserole dish; smooth out top. Starting at the top left, alternate one marshmallow with two pecan halves until the entire casserole is covered. Bake at 350° F for 25 to 30 minutes, or until marshmallows are browned and puffed.