Cook bacon in a heavy deep skillet until crispy. Remove and drain on paper towels, leaving drippings in skillet.
Add shallots and garlic to skillet and cook over medium heat until tender, about 3 minutes.
Meanwhile, cook pasta according to package directions and drain, saving a cup of the pasta water right before draining.
Pour pasta into skillet along with about half the pasta water and 1/3 cup of the parmesan cheese. Toss over low heat until everything is well mixed.
Add the egg to the pasta mixture and toss again. Add the lemon zest and juice and the rest of the cheese and toss again. Drizzle in a little more pasta water if needed to make a light sauce.
Add the bacon, the butter and a grinding of pepper, and toss again. Divide among warmed plates. Garnish with lemon peel and serve!