Bring a large pot of salted water to a boil. Pulse the bread, garlic and parsley in a food processor to form moist, green crumbs.
Cook the bucatini in the boiling water as the label directs. Reserve 1/4 cup cooking water, then drain.
Meanwhile, melt the butter in a wide pot over medium heat. Add the prosciutto and shallots and cook until the shallots are tender and beginning to brown, about 6 minutes. Add half of the breadcrumbs and cook, stirring, 2 minutes. Add the wine and the reserved cooking water and cook, uncovered, until the liquid is almost completely reduced, about 2 minutes. Season with pepper.
Add the mussels to the pot, cover and cook, shaking the pot occasionally, until the mussels open, about 6 minutes (discard any that do not open). Add the pasta and heat through, tossing to coat. Divide among bowls and top with the remaining breadcrumbs.