Preheat the oven to 220 C / Gas 7. Grease a roasting tin.
Pierce the sweet potatoes and regular potatoes all over with a fork. Place potatoes in the prepared tin.
Bake potatoes in the preheated oven for 1 hour, or until tender.
Remove potatoes and cut in half immediately. Use a tea towel and/or gloves to protect your hands since the potatoes will be hot. You want as much steam and moisture to escape to get a lighter gnocchi.
Lightly dust your work surface. Scoop out the potato flesh and pass through a ricer directly onto the floured work surface. If you don't have a potato ricer, use a grater.
Sprinkle the potatoes with the grated cheese, salt and pepper. Taste and add more seasoning if needed.
Drizzle the beaten egg yolk over the potatoes. Knead briefly to incorporate. Start sifting flour over the potatoes, gradually adding a few tablespoons at a time. Stop adding flour when the potato mixture is smooth and slightly sticky. You may not need all of the flour. Form the dough into a ball, and sprinkle the top of the ball with flour. Remember the less flour you use the better.
Form the dough into five smaller balls, and lightly dust with flour. Roll each ball into a 1 to 1.5cm rope, then cut into 1 to 1.5cm pieces. As you work, transfer the gnocchi to a tray lined with baking parchment. Repeat with remaining ropes.
Bring a large pot of salted water to the boil.
Make the traditional grooves in the gnocchi by using a gnocchi board, or use the back of the tines of a fork.
When the water is at a rolling boil, add gnocchi in batches. Once the gnocchi rise to the top, after 3 to 4 minutes, remove the gnocchi with a slotted spoon and transfer back to the tray.
Now make the sauce: place a large frying pan over medium low heat. Add the nduja and cook till the fat has rendered, 3 to 5 minutes. Add butter to the pan and stir. When the butter begins to foam, add the sage. Allow the sage to fry and the butter to brown. Stir constantly to make sure the butter doesn't burn. Once the butter is browned to your liking, remove from heat and let cool for a couple of minutes. Remove the sage from the pan and set aside. Stir balsamic vinegar into the sauce.
Place the pan back over low heat, then add the gnocchi. Heat the gnocchi through over low heat for about 5 minutes. Make sure to stir constantly so the gnocchi gets coated evenly with the sauce.
Plate six portions of gnocchi and serve each drizzled with a tablespoon of balsamic glaze, then top with grated cheese and the fried sage leaves.