Cream of Cauliflower Soup with Crispy Prosciutto and Parmesan

2 large cauliflowers (about 2 pounds each)
4 tablespoons unsalted butter
3 cups chopped leeks, white and light-green parts only (3 to 4 medium leeks)
8 cups chicken stock
1/8 teaspoon cayenne pepper
1 cup grated Parmesan cheese, preferably Parmigiano-Reggiano, divided
½ cup creme fraiche
Salt, preferably kosher
1/4 pound prosciutto, thinly sliced
1 tablespoon olive oil
½ cup chopped flat-leaf parsley

Cut cauliflowers into florets to yield about 12 cups.

Melt butter in a large, heavy, deep-sided pot (with a lid) over medium-high heat.

Add leeks and saute, stirring until softened, 4 to 5 minutes. Add cauliflower florets, chicken stock and cayenne pepper. Bring to a simmer, stirring occasionally, then reduce heat to a gentle simmer and cover pot with a lid. Cook until vegetables are very tender, about 20 minutes.

Puree soup in the pan using a stick blender or transfer it in batches to a food processor or blender. Whisk in half of the cheese and all of the creme fraiche. Season soup with salt to taste, if needed.

Slice prosciutto into strips about 3 to 4 inches long and 1/4 inch wide. Heat olive oil in a medium skillet over medium heat.
When hot, add prosciutto and saute, stirring constantly, until crisp and browned, 4 to 5 minutes. Remove and drain on paper towels.