Heat the butter in a heavy-based frying pan over a medium-high heat until melted then add the bacon and let it cook for 2-3 minutes. Add the cloves of garlic and stir well.
Meanwhile, cook the spaghetti in a large saucepan of boiling salted water according to packet instructions until al dente; usually 8-10 minutes.
As the spaghetti is cooking, reduce the heat of the frying pan and reduce the garlic cloves. Whisk the eggs with black pepper and most of the parmesan, keeping a little to garnish with.
Drain the pasta, keeping the cooking water, and add to the pancetta. Add the cheese and egg mixture to the spaghetti in the pan immediately and and toss everything together, lifting well and stirring to coat evenly. Make sure the spaghetti is piping hot to ensure the egg is cooked through.
Season to taste.
If the sauce is a little dry, add a little of the cooking water to loosen it up. Transfer to serving bowls and garnish with the remaining parmesan sprinkled over and a few bay leaves on top.
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