Cook spinach in large pot of boiling salted water just until wilted, stirring occasionally, about 2 minutes.
Drain. Squeeze out liquid. Chop spinach.
Mix spinach, ricotta, 1/2 cup Parmesan, 1/2 cup flour, egg yolks, salt, pepper and nutmeg in bowl until slightly sticky dough forms.
Dust baking sheet with flour. Working in batches and using floured hands, roll 1/4 cup dough on floured work surface to form 5-inch-long rope.
Cut rope into 1-inch pieces.
Roll each piece between palms to form oval.
Transfer gnocchi to prepared baking sheet. Repeat rolling, cutting and shaping with remaining dough. Working in batches, add gnocchi to pot of boiling salted water; cook until gnocchi rise to surface. Cook 4 minutes longer. Using slotted spoon, remove gnocchi from water; drain.
Place in serving dish.
Pour butter over gnocchi; toss. (Can be made 1 day ahead. Cover; chill. Reheat in 400°F oven about 10 minutes.)
Sprinkle with 1/2 cup Parmesan. Season with salt and pepper.
