sweet potato gnocchi recipe

Ingredients: 2 large sweet potatoes 1 large egg, lightly beaten 1/2 cup ricotta cheese 1/2 cup grated Parmesan cheese 1 teaspoon salt 1/4 teaspoon ground nutmeg 2 to 2 1/2 cups all-purpose flour (plus more for dusting) 2 tablespoons butter Fresh sage leaves (optional) Grated Parmesan cheese (for serving)