Meanwhile, in a large skillet over medium high heat, add the bacon and cook until brown and crispy, about 5 to 7 minutes. Reserve 2 tablespoon bacon fat and discard remainder. Add garlic and cook, stirring, 1 minute or until fragrant. Remove skillet from heat.
In a medium bowl, whisk together eggs and Parmigiano-Reggiano cheese; set aside.
Cook pasta according to package directions. Before draining, set aside 1 cup pasta water; drain pasta. Whisk ¼ cup reserved pasta water into egg mixture,
Transfer pasta to the skillet and toss with the bacon fat. Very slowly stream egg mixture in skillet, tossing constantly, about 3 to 5 minutes or until cheese is melted and egg is thickened. Season to taste with salt and pepper. Add reserved pasta water, one tablespoon at a time, until desired consistency is reached.
Serve garnished with parsley, if using.
Thick cut of bacon, when diced, makes for the ideal sized pieces so every mouthful has bacon it it. Pancetta also works well.
You can omit the egg yolks, if preferred, and stir in a little heavy cream to give it the consistency you desire.
Work quickly to stir the egg mixture into the warm skillet to make sure the eggs don't start to cook before the sauce forms.
Feel free to use shredded parmesan cheese in place of the Parmigiano-Reggiano Cheese.