Whisking constantly, add about 1 cup pasta cooking liquid to egg mixture to warm it up. Add egg mixture to pasta and cook, tossing constantly, keeping the mixture below a simmer, and adding more pasta cooking liquid by the ½-cupful if needed to adjust the consistency, until sauce thickens just enough to lightly coat pasta, about 5 minutes. Do not rush this step or you risk ending up with a pan of pasta and scrambled eggs. Remove skillet from heat if you want to slow the process down. As soon as you see wisps of steam rising from the egg mixture, that’s a good indication that the sauce is about to go from very liquidy to perfectly thickened. Remove from heat and toss in guanciale.
