How to Make It
Dice the prosciutto and transfer to a bowl. Freeze until very firm, 30 minutes. In a food processor, pulse the chilled prosciutto until finely chopped, about 30 seconds. Return to the bowl.
Meanwhile, in a pot of salted boiling water, blanch the spinach until just tender, about 1 minute. Transfer to a colander and let cool slightly. Press out all of the excess water and coarsely chop the spinach; you should have 1/2 cup.
In a medium bowl, whisk the ricotta until smooth. Add the prosciutto along with the chopped spinach, the mascarpone, egg yolks and Parmesan; stir well. Season with salt. Cover the filling with plastic wrap and refrigerate until firm, about 30 minutes.
In a large bowl, whisk the 00 flour with the salt; make a well in the center. Add the egg yolks and 1 tablespoon of water to the well and mix. Using a fork, gradually incorporate the flour into the wet ingredients until a shaggy dough forms. Scrape the dough out onto a work surface very lightly dusted with semolina flour and knead until stiff but smooth, 5 minutes. Wrap in plastic and let rest at room temperature until softened, 45 minutes.
Generously dust a work surface with semolina flour. Line a rimmed baking sheet with parchment paper and dust with semolina flour. Divide the pasta dough into 4 pieces and work with one at a time; keep the rest covered. Press the dough to flatten. Using a hand-cranked pasta machine and starting at the widest setting, run the dough twice through each of the first 5 settings, then run it once through the sixth setting. Cut the sheet in half; run each half through the thinnest setting one time. Transfer the sheets to the prepared work surface.
Lay 1 pasta sheet on a work surface with a long edge facing you. Spoon eight 1-teaspoon-size balls of filling in each of 2 rows on the sheet, allowing 1 inch of space between them. Very lightly brush the dough around the filling with water. Place the second pasta sheet on top. Using your fingers, press the dough around each mound of filling. Using a 2-inch fluted cutter, stamp out the ravioli; transfer to the semolina-dusted baking sheet. Cover with a clean tea towel. Repeat with the remaining dough and filling to make 64 ravioli.
In a pot of salted boiling water, cook the ravioli in 2 batches until the edges are tender, about 2 minutes. Using a slotted spoon, transfer to a shallow bowl. Reserve 1 cup of the cooking water.
Melt 4 tablespoons of the butter in each of 2 large skillets. Add 1/2 cup of the pasta water and half of the chopped spinach to each skillet and cook over moderately high heat until the spinach is just wilted, about 1 minute. Add half of the ravioli to each skillet and cook, stirring gently, until they are coated in butter. Using a slotted spoon, transfer the ravioli and spinach to a large serving bowl. Scrape the remaining liquid from 1 large skillet into the other. Add the remaining 2 tablespoons of butter to the skillet along with 1 teaspoon of the fennel pollen and 1/4 cup of the cheese. Cook over moderate heat, swirling the pan gently, until the butter has melted and the sauce has thickened slightly, about 1 minute. Scrape the sauce over the ravioli. Sprinkle with the remaining fennel pollen and cheese and serve immediately.
Doppio zero (“double zero,” or 00) is a fine Italian flour available at specialty food shops and from amazon.com.