Wash, peel, and chop the sweet potatoes into even-sized cubes, about 1-inch in size.
Add the peeled and chopped sweet potatoes to a large pot. Fill the pot with cold water and salt the water with 1/2 teaspoon to 1 teaspoon of salt.
Bring the sweet potatoes to a boil. Once boiling, cook them for 15-20 minutes, or until fork-tender.
Place the boiled sweet potatoes in a strainer to drain them.
Add the drained sweet potatoes back to the pot. Use a potato masher to mash the sweet potatoes.
Add the milk, maple syrup, pumpkin pie spice, cinnamon, applesauce, vanilla extract, salt, and pepper to the pot. Mix to combine these ingredients.
Lightly spray a 9x9 baking dish with oil. Then, transfer the sweet potato mixture to the baking dish. After that, sprinkle the mini marshmallows and optional chopped pecans on top.
Bake the sweet potato casserole at 350°F for 15-20 minutes.