Special equipment: a plastic piping bag or resealable plastic bag; a paring knife or fluted pasta cutter
Melt 2 tablespoons butter in a medium skillet over medium heat until foamy. Add the garlic and cook until it starts to become golden, about 2 minutes. Stir in the spinach and 1/4 cup water. Cook, stirring often, until the spinach is tender and the liquid has evaporated, about 5 minutes. Transfer to a food processor and add the ricotta, mozzarella, Parmesan and 1/2 teaspoon each salt and pepper. Puree until smooth and bright green. Spoon into a plastic pastry bag or large resealable plastic bag, snip the tip to 1/4 inch wide and set aside.
Put 8 pieces of pasta on a clean work surface and pipe a circle of the spinach mixture in the middle of each square, making sure to leave about a 1-inch border all around. Using the back of a spoon, make a well in the middle deep enough to hold an egg yolk. One by one, separate 8 eggs and put a yolk in each well. (Reserve the whites for another use.)
Crack the remaining egg into a small bowl and beat lightly with a fork. Brush the edges of the pasta pieces with the egg, and then top with the remaining squares of pasta. Working from the edge of the filling outward, firmly press the dough together, making sure there are no air bubbles. Trim the edges of each ravioli with a sharp knife or a fluted pasta cutter.
Bring a large pot of salted water to a boil. Melt the remaining 6 tablespoons butter in a small skillet with the sage leaves over medium heat and cook, swirling often, until the butter is golden brown and the sage is fried and crisp, about 3 minutes. Set aside. Gently drop the ravioli into the water and cook until the pasta is tender and the egg is still runny, about 6 minutes.
Remove from the water with a slotted spoon or handheld strainer and place each ravioli in an individual bowl. Top each with a sage leaf and evenly spoon the butter over. Serve immediately.
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