Preheat oven to 500. Heat a lg deep skillet over med-high heat and cook 8 slices bacon, cut into one inch pieces until crispy. Remove & drain on paper towels, but leave the drippings in the pan. Add 2 leeks, rinsed and sliced (white and light green parts only) and saute until tender, about 3-5 min. Add 1 1/2 cups white wine and simmer for a minute or two. Place 4 lbs mussels in large deep roasting pan and pour the wine sauce over them. Cover the pan securely with foil and bake for 15 minutes or until mussels are open. Push mussels to the side and stir in the butter, if using. Sprinkle all with salt. Stir in bacon and scatter parsley on top.