Cut peeled sweet potatoes into 2-inch chunks. Place them in a large pot, cover with lightly salted water, and bring to a boil. Simmer, covered, until tender, about 15 minutes. Drain. Reserve 3 chunks (about 1/2 potato) for another use. Mash the remaining pieces with 3 tablespoons maple syrup, 1/3 cup low-fat milk, 2 tablespoons unsalted butter, and 1 teaspoon kosher salt.