In a Dutch oven, combine the squash, ⅓ cup water, olive oil, and a scant ½ teaspoon salt and bring to a boil over high heat. Cover and cook until squash is just tender and water has evaporated, 5 to 6 minutes. Stir in the garlic and rosemary and sauté until fragrant, about 1 minute. Transfer the mixture to a shallow bowl and mash the squash with a fork until mostly smooth. Stir in the prosciutto and cheese.
Wash out the pot and fill with a generous 2 quarts of water. Add 1 tablespoon of salt and bring to a simmer.
Set out a large wire rack and a small bowl of water. Lay 6 wonton wrappers on a dry work surface. Brush the edges with water, then drop 1 tablespoon of filling onto each wrapper. Working with 1 at a time, top each wrapper with another wrapper; press to seal completely. Transfer to a wire rack. Repeat with the remaining wrappers and filling, keeping the work surface dry.
MAKE THE ROSEMARY OIL:
Heat the oil and rosemary in a small skillet over low heat until the rosemary starts to sizzle. Remove from heat.
COOK THE RAVIOLI:
Gently slide 12 ravioli into simmering water. Cook until the wrappers start to wrinkle and turn from opaque to translucent, 3 to 4 minutes. With a slotted spoon, remove the ravioli and arrange 6 on each plate.
Drizzle with 2 teaspoons cooking water and 1 teaspoon rosemary oil. Sprinkle with pepper and Parmesan cheese, and serve. Cook the remaining 12 ravioli, and serve them as soon as they are ready.
Notes
You'll get 8 servings (3 ravioli each) if you prefer to serve these as an appetizer.