Preheat oven to 350 degrees F (175 degrees C). Lightly grease a baking 9x13 inch baking dish.
Place the sweet potatoes in a large pot, and fill with enough water to cover. Bring to a boil, and cook until a fork easily pierces the potatoes, about 10 minutes.
Drain, and cool.
Mix the brown sugar, cinnamon, ginger, nutmeg, and salt together in a bowl. Stir in the cashews.
Arrange half the sweet potatoes over the bottom of the prepared baking dish.
Sprinkle with half the brown sugar mixture. Top with half the butter pieces. Repeat with a second layer of sweet potatoes, brown sugar mixture, and remaining butter pieces.
Bake in preheated oven until potatoes are glazed with the brown sugar mixture, about 30 minutes.
Sprinkle mini marshmallows over the top, and return to the oven to bake until browned, about 5 minutes more.
