1. Preheat oven to 375 degrees. Butter a 1 1/2-quart souffle dish; dust with sugar.

2. In a medium saucepan, heat butter over medium. Add flour, and cook, stirring, until golden, about 2 minutes. Gradually whisk in milk; simmer, whisking constantly, until thickened, about 1 minute. Remove from heat; mix in sweet potatoes and egg yolks. Stir in maple syrup and nutmeg, and set aside.

3. Using an electric mixer, beat egg whites with a pinch of salt until stiff peaks form. Whisk 1/4 of whites into sweet potato mixture; using a rubber spatula, gently fold in remaining whites. Pour mixture into prepared dish; place on a rimmed baking sheet. Bake until puffed, 35 to 45 minutes. Serve immediately, with whipped cream, if desired.