Remove and discard the root, tough outer leaves, and tops of leeks to where the dark green begins to pale. Chop leeks, and saute in butter in a large Dutch oven for 5 minutes or until tender.
Add squash, and saute for 4 minutes. Stir in chicken broth. Bring to a boil, cover, reduce heat and simmer for 15 minutes or until the squash is tender. Add the watercress, hot sauce, and salt and pepper to taste, and simmer for 2 minutes.
Transfer mixture in batches to a blender (I use a handheld blender and leave everything in the pot). Ladle soup into soup bowls. Garnish, if desired.