Preheat oven to 350 degrees F. Pierce each sweet potato in several places with a fork. Place directly on the oven rack and roast until soft, 45 minutes to 1 hour. Transfer to a cutting board; let stand until cool enough to handle, about 10 minutes. Slip off the skins and cut the sweet potatoes into 1-inch slices; transfer to a large bowl. Add butter. Smash the sweet potatoes with a potato masher or fork until fluffy but some lumps remain. Add maple syrup, chili powder, ground cumin, ginger, salt and pepper; stir to combine.