Preheat oven to 375 F. Butter a 1 1/2 quart souffle dish, and dust with sugar.
In a medium saucepan, heat butter over medium heat. Add flour, and stir until golden, about 2 minutes. Gradually whisk in milk and simmer, whisking constantly until thickened, about 1 minutes.
Remove from heat, and mix in sweet potato and egg yolks. Stir in maple syrup and nutmeg, and set aside.
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, placed on a rimmed baking sheet. Bake until puffed, 35-45 minutes. Serve immediately, with whipped cream if desired.