Cook potatoes in their jackets in a little water with the lid on,virtually steaming them.
Peel while still hot then pass through a ricer and cool. Do not use a blender.
Make a well in centre of potato mound and add a handful of flour,the yolks,salt,nutmeg and 50g parmesan. Fold continuously towards centre,gradually adding more flour until it comes together without being sticky. Allow to rest for 5 minutes.
Shape by rolling some mixture into a sausage shape about 3cm in diameter and cutting off 3cm pieces. Roll each piece on the back of a fork using thumb and forefinger. Once a few have been shaped,test by bringing a small saucepan of water to boil.
Add three or four gnocchi. When they rise,check they aren't too soft or falling apart. If they are,add more flour. To serve,poach gnocchi in a large pot of salted boiling water until they rise to the surface,then scoop on to a large serving plate.
Meanwhile,in a small pot,heat butter on high until it develops a nut-brown colour. Add sage and continue cooking for 30 seconds until leaves are crisp. Sprinkle remaining parmesan and drizzle sizzling butter on to gnocchi.