Preheat the oven to 200C (400F). Prick the sweet potato with a fork and bake in the preheated oven until soft (about 45 minutes). Remove the sweet potato flesh from the skins and mash the flesh into a puree.
Put 2 cups of the puree into a bowl or a mixer. Add the salt, egg and garlic powder (or crushed garlic) and mix. Add the flour 1 cup at a time, mixing after each addition, until the dough is soft but not too sticky and forms a ball (it will be a semi-wet mixture).
Tip the dough onto a well-floured surface and form it into 6 balls. Roll each ball out into a log, then, using a sharp knife, cut each log into 3cm x 3cm pillows of gnocchi. If desired, carefully roll a fork over each piece to make grooves.
Bring a large pot of water to the boil. Add the gnocchi to the boiling water and stir gently. Cook until the gnocchi starts to float, 3-5 minutes. Drain well.
Meanwhile, melt the butter in a frypan over a medium heat. Let it start bubbling and browning, being careful not to burn it. It should brown slowly and turn a caramel colour quite quickly.
Add the gnocchi to the frypan and turn in the butter until they are brown and crispy on all sides. Add the sage leaves in at the last minute and toss through.
Transfer onto a serving plate and drizzle the extra butter from the pan over the top. Sprinkle the parmesan over, grind a little black pepper and sea salt over top, and serve immediately.