Cook potatoes and once cooked let cool down completely (I cooked the night before and left on the kitchen counter until next morning).
Rise potatoes or mash until no clumps are in there anymore.
Sieve about 3/4 cup of flour onto work surface.
Dump potato mash on flour.
Whisk egg yolks and dump onto potato mash.
Sieve another 3/4 cup of flour onto potato mash.
Knead the ingredients until JUST combined. Do not overwork. If the dough is very sticky add more flour but only a tablespoon at a time in order not to add too much. As soon as you are able to form a dough that won't stick to your hands. Stop kneading immediately and don't add any more flour.
Cut dough into 4 pieces.
Flour new work surface and roll out one of the four pieces into a long sausage. Flour your hands if necessary to avoid sticking. Be very gentle when rolling in order not to flatten dough.
Cut thumb thick pieces off the sausage and press your index finger into the middle to achieve an indentation.
Place gnocchi on a lined baking sheet with enough space in between them in order to avoid sticking together.
Heat about 4 l of salted water in a large pot and once boiling add 15-20 gnocchi at a time and cook until they float.
Spoon out and place in a sieve to drain remaining water.
Continue last two steps until all gnocchi are cooked.
Heat a skillet over medium heat and once hot add about 1 tablespoon of butter per serving. Once sizzling, add sage leaves (3-5 per serving) and after about 30 seconds add 15-20 gnocchi (per serving) and salt in the pan.
Once brown on one side turn one by one to brown from the other side.
You can safe all or part of the cooked gnocchi in the fridge for the next day or to prepare another recipe if not eaten in sage butter right away.