In a small bowl, whisk together ½ cup heavy cream, 1 of the egg yolks, grated parmesan & pecorino, finely grated, and 1 tsp freshly ground black pepper, plus extra to serve. Set aside.
Cook pancetta in a large pan over medium heat with a splash of neutral oil (or even a tablespoon of butter) until the edges are crisp and the fat has rendered. Remove the pancetta and set aside, leaving 1–2 tbsp of fat in the pan.
Cook 2 frozen udon noodles (thick Sanuki-style) according to packet instructions (usually just a quick boil). Reserve ½ cup reserved udon cooking water of the hot cooking water before draining. Drain and leave aside - do not rinse the noodles.
Add 1 tbsp unsalted butter to the pan with the pancetta fat over low heat. Add the drained udon and toss to coat. Turn the heat off, then pour in the sauce. Toss gently, adding splashes of the reserved cooking water until the sauce is glossy, pale, and coats every noodle. Add half the pancetta back in and toss once more.
Divide into warm bowls. Top with the remaining pancetta, extra grated parmesan, a generous crack of black pepper, a pat of butter, and one raw egg yolk per bowl. Mix everything together at the table just before eating. Serve immediately — the sauce will dry out if it sits.