Prepare the Sweet Potatoes: Preheat your oven to 400°F (200°C). Pierce the sweet potatoes with a fork and bake them for about 45 minutes, or until they are tender. Once cooled, peel and mash them in a large bowl until smooth.
Make the Gnocchi Dough: Add the egg and salt to the mashed sweet potatoes and mix well. Gradually add the flour, ½ cup at a time, mixing until a soft dough forms. The dough should be slightly sticky but firm enough to handle.
Shape the Gnocchi: Transfer the dough to a floured surface. Divide it into 4 sections and roll each section into a long rope about 1 inch thick. Cut the rope into 1-inch pieces. Optionally, roll each piece over the back of a fork to create ridges.
Cook the Gnocchi: Bring a large pot of salted water to a boil. Working in batches, drop the gnocchi into the boiling water. When they float to the surface (after about 2-3 minutes), use a slotted spoon to remove them and set aside.
Make the Brown Butter Sauce: In a large skillet, melt the butter over medium heat. Add the sage leaves and cook until the butter turns golden brown and the sage leaves crisp up, about 3-4 minutes. Season with salt and pepper.
Toss the Gnocchi: Add the cooked gnocchi to the skillet with the brown butter and sage. Gently toss to coat the gnocchi in the sauce. Cook for another 2-3 minutes to allow the flavors to meld together.
Serve: Serve the gnocchi hot, with freshly grated Parmesan cheese on top, if desired. Enjoy!