Heat grill pan over high heat.
Coat the chicken in extra-virgin olive oil and season with salt and pepper. Grill about 12 minutes, turning occasionally, then set aside.
Bring water in saucepot to a boil for the pasta. Salt the water, add pasta and undercook by 1 minute.
While the pasta cooks, melt 6 tablespoons butter in a separate saucepot. Spoon out 1/2 and toss with the panko to moisten. Combine with 1/2 the grated Pecorino and reserve. To the remaining melted butter, add the garlic and anchovy paste, lots of coarse black pepper, stir 2 minutes then whisk in flour then stock, milk, and Worcestershire, thicken until it coats a spoon, then stir in the remaining half of the grated Pecorino.
Drain the pasta. Halve the chicken breasts lengthwise, then thinly slice and combine with the pasta and sauce, transfer to a casserole and top with the bread crumbs. Cool and store for make-ahead meal.
To reheat: Reheat, covered, in a hot 425 degree F oven to warm through, then uncover to brown.
Serve with shredded escarole or romaine and dress with lemon juice and extra-virgin olive oil. Serve alongside Chicken Caesar Mac.