Blanch the spinach in boiling water until just wilted, then drain, plunge into iced water and drain again. Squeeze out as much liquid as possible, then chop finely and put into a bowl.
Add the ricotta, Parmesan and the egg yolks and season with salt and pepper to taste. Roll level tablespoons of the mixture into balls, place on a tray and refrigerate for 20 minutes.
Bring a large saucepan of lightly salted water to a gentle boil. Remove the gnocchi from the refrigerator and lightly dust with flour. Add the gnocchi to the water, a few at a time, and remove with a slotted spoon as they rise to the surface.
Divide the gnocchi among four bowls and sprinkle liberally with Parmesan and freshly ground pepper.
Meanwhile, put the butter into a frying pan with the sage leaves and heat until the butter turns nut brown. Spoon the butter and sage over the gnocchi and serve immediately.