Cut the sweet potatoes in half lenghtwise and place cut side down on a baking sheet. Bake in a preheated oven at 200°C (400°F) for about 20 minutes, or until tender.
Alternatively, boil the sweet potatoes (whole or in big chunks) for about 10 minutes.
Scoop the sweet potato flesh in a bowl when it's still hot and add the salt and chili powder to taste. Then add the flour and mix well with a wooden spoon for a few minutes.
When you have a dough consistency, knead with your hands for a few more minutes until you have a uniform dough. Add more flour if it's too sticky although it's not recommended as it will make the gnocchi harder.
Let the dough cool down, then dust flour on a wooden board and roll a piece of gnocchi dough into a long snake about ½ inch (1-2 cm) thick.
Cut little pieces of dough about ¾ inch (2 cm) long and then roll them on a gnocchi board or on a fork to give them texture.
Set the gnocchi on a clean towel and dust will a little bit of flour so they don't stick together.
Bring a large pot of salted water to boil and add the butter and sage on a large pan, melting the butter at low heat.
Toss the gnocchi in the boiling water and once they start floating to the surface, drain them and toss in the pan with the melted sage butter. Fry the gnocchi in the butter for a minute and serve immediately. Sprinkle with Parmigiano Reggiano or Pecorino to taste.
Notes
You can use other dry spices to give the spiciness, from cayenne pepper to paprika. I suggest to add the spices to the potatoes before you add the flour, so you can taste the amount of spiciness and adjust. Remember that the flour will tone it down a little.