Preheat oven to 350°F. Butter an 8-inch baking dish. Place potatoes and 1 tsp. salt in a large pot and cover with water; bring to a boil. Cook until potatoes are tender, 15 minutes. Drain.
Place potatoes in a large bowl and whip with an electric mixer until smooth and light. Add sugar, eggs, cinnamon, nutmeg, 1/2 tsp. salt, evaporated milk and butter; beat until smooth. Pour sweet potato mixture into prepared dish and bake until golden and slightly puffed, 35 to 40 minutes. Let stand 10 minutes, then serve warm.