Melt the butter in a big soup pot, set over med-high heat.
Add in leeks; saute/stirring, until softened, about 4-5 minutes.
Add in cauliflower florets, chicken stock, and cayenne pepper; bring to a simmer, stirring occasionally; then reduce heat to a gentle simmer and cover pot.
Cook until vegetables are very tender, about 20 minutes.
Puree the soup in batches in a food processor, blender, or food mill, and return soup to pot (or use an immersion blender to puree the soup in the pot).
Whisk in 1/2 cup of the cheese and the creme fraiche; taste the soup and season with salt if needed (the soup can be made 2 days ahead; cool, cover, and refrigerate; reheat over medium heat).
Cut the prosciutto into julienne strips 3-4 inches long and 1/4 inch wide.
Heat the olive oil in a medium skillet over medium heat; when hot, add the prosciutto, saute/stir occasionally, until crisp and browned, for 4-5 minutes.
Using slotted spoon, transfer prosciutto to paper towels to drain.
To serve--ladle soup into 6 soup bowls; sprinkle each serving with some prosciutto, chopped parsley, and remaining 1/2 cup Parmesan.