Add the sweet potatoes to the air fryer and air fry at 200 degrees Celsius (390 Fahrenheit) for 35-40 minutes or until the fresh is soft.
Alternatively, roast them in the oven at the same temperature, cooking time can be longer, so check them after 40 minutes.
Once cooked, allow them to cool down completely, then peel and mash them well.
Sift the flour on5o the work surface, make a well in the middle and crack in the egg.
Use a fork to beat the egg well, pulling in flour from the sides as you beat.
Add the mashed sweet potatoes and grated cheese, and knead everything well until you get a dough that is smooth and elastic, and just very slightly tacky.
Dust the work surface with more flour and divide the dough into several portions.
Shape each portion into a long rope of 2 cm/ 1 inch thickness, then cut each rope into bite-sized pieces.
Roll each piece over the back of a fork or a gnocchi board to create ridges.
Bring a pot of salted water to the boil, and working in batches, add the gnocchi.
Cook them for 2-3 minutes or until they float to the surface.
Once they float, cook for a further 1 minute then remove them using a splotted spoon.
To make the sauce, melt the butter in large frying pan set over a low to medium heat.
Add the sage leaves and cook until the butter is golden and smells nutty.
Add the gnocchi to the sauce, and toss gently to coat.
Cook for 1-2 minutes, then serve with plenty of grated parmesan.