Place spinach in a steamer basket set over simmering water. Cook until spinach is wilted; drain. Heat 1/2 tablespoon butter in a medium skillet over medium-high heat. Add spinach and cook until coated with butter, about 2 minutes. Let cool completely.
Finely chop spinach and add to a medium bowl along with ricotta and half the Parmigiano-Reggiano. Season with nutmeg, salt, and white pepper. Stir to combine. Transfer filling mixture to a pastry bag fitted with a 1/2-inch round plain tip; set aside.
Bring a large pot of water to a boil. Set the rollers of a hand-cranked or electric pasta machine at their widest opening. Flatten 1 piece dough with the palms of your hands; dust lightly with flour. Roll 1 piece of dough through widest setting. Keep rolling dough through machine on decreasing settings until you have rolled it through the first 6 settings. Repeat with 1 more piece of dough.
Lay 1 piece of dough flat on work surface. Using a 2-inch round cutter, gently mark 4 circles in dough, about 5 inches apart. Using circle as guide, pipe filling around each circle and continue piping to form a 1-inch-high edge. Place 1 egg yolk in the center of each circle; season with salt and white pepper. Brush a little cold water around filling and gently place second piece of dough over filling and yolks to cover. Gently press edges together to seal, eliminating as much air as possible. Center a 4-inch fluted round cutter around filling and cut each raviolo.
Add salt to boiling water and return to boil. Gently transfer ravioli to boiling water and cook for 2 minutes. Meanwhile, place remaining butter in a small saucepan over medium-high heat. Cook until melted and lightly browned. Drain ravioli and divide among 4 serving plates. Pour browned butter over ravioli and garnish with remaining cheese and shaved truffles.