Preheat the oven to 350 degrees F.
Bring a large pot salted water to a boil.
Cut the stems off the spinach, and soak the leaves in the sink. Wash them thoroughly.
Add the spinach to the boiling water and cook 1 minute. Remove spinach from the water and plunge in a large bowl of ice water to stop the cooking.
Drain the spinach very well, and using a clean tea towel, squeeze out the extra water. Chop the leaves very fine.
Put the cooked spinach in a large bowl, add the ricotta, 3 tablespoons Parmesan, egg yolks, and season with a couple pinches salt and some freshly ground black pepper. Mix well, and make sure that the ricotta crumbles into fine pieces, and mixes properly with the spinach. Shape the mixture into balls about the size of walnuts.
Lightly butter a 13 by 9-inch flameproof casserole dish, lay all the gnocchi in it, and dress it with a few very thin slices butter and a sprinkling of Parmesan. Cook the gnocchi at 350 degrees F for about 20 minutes, and then use the broiler for the last 5 minutes to crisp and brown the top of the spinach balls.
In a medium skillet, melt the remaining 1 1/2 tablespoons butter, and saute the sage until it starts to brown. Serve the gnocchi on a warm plate and dress with the sage-butter sauce, and garnish with freshly grated Parmesan.