Step 1

Bring a large pot of water to a boil. Fill a large pot with at least 4 quarts of water and bring to a boil over medium-high heat.

Step 2

Heat the oven to 375°F and butter a casserole dish. Arrange a rack in the middle of the oven and heat the oven to 375°F. Coat a 9x9-inch square baking dish or other 2-quart casserole dish with butter; set aside.

Step 3

Boil the sweet potatoes until tender, about 25 minutes. Add the sweet potatoes to the boiling water and then reduce heat to medium. Simmer until completely tender, about 25 minutes.

Step 4

Drain the sweet potatoes and mash. Drain the sweet potatoes, then transfer them to a large bowl. Add the brown sugar, butter, pumpkin pie spice, and salt. Using a potato masher, mash to desired consistency, folding in the sugar and butter as you go.

Step 5

Top with mini marshmallows and bake. Transfer the mashed sweet potatoes to the prepared baking dish and smooth into an even layer. Sprinkle with the mini marshmallows. Bake until the top is golden-brown and crisp, 20 to 25 minutes.