Heat a large pot or Dutch oven over med/high heat and add the chopped pancetta. Sauté, stirring occasionally, until browned and crisp, then transfer to a paper towel-lined plate. Keep 3 Tbsp of bacon grease in the pot (discard excess if desired).
Add veggies - toss in chopped leeks, onion, and celery and continue cooking over med/high heat, stirring occasionally until soft and golden (7-8 min). Sprinkle in flour and saute another minute, stirring constantly.
Build Broth - Add chicken broth, ray leaf, Worcestershire, Tabasco sauce, thyme, salt, and black pepper. Bring the soup to a light boil.
Add Potatoes - While the soup is heating up, chop potatoes into bite-sized pieces about 1/4" thick. Add potatoes to pot, then stir in milk and whipping cream. Bring to a boil, reduce heat, cover, and lightly boil until potatoes are easily pierced with a fork (about 20 min).
Add Clams - When potatoes are tender, add chopped clams, return to a light boil, and remove from heat. Don't overcook clams or they will get tough*. Season to taste with salt and pepper. To serve, garnish with cooked bacon and fresh cilantro if desired.