Peel and cut 2 pounds sweet potatoes into 1-inch pieces. Bring 4 cups water to a boil in a large pot over medium-high heat. Add 1 cup milk, 1 stick unsalted butter, 1 teaspoon ground cinnamon, and 1 teaspoon kosher salt. Carefully add the sweet potatoes. Reduce the heat to medium and boil, stirring occasionally, until the potatoes are tender, 15 to 17 minutes.