Put the butter, olive oil, onion, celery, carrot and pancetta into the pan and cook, stirring, over medium heat until they start to soften and turn translucent.
Add the meat and chicken livers (if using), crumbling the meat as you drop it into the pan, and a pinch of salt and cook, breaking up lumps in the ground meat with a wooden spoon, until all the pink has gone.
Add the wine and allow it to bubble for a few minutes, then add the passata. Let everything bubble for 2 minutes before turning the heat down to the ragù cooks at the barest simmer, with just the odd blip bursting to the surface, for no less than 2 hours.
During cooking, lift the lid every now and then and add the milk bit by bit. By the end of cooking, the ragù should be rich and thick.
Bring a pot of water to a boil, add salt, stir, then drop in the tagliatelle and cook until al dente.
Meanwhile, put half the hot ragù into a large, wide, warm bowl. When the pasta is al dente, drain and tip into the bowl, add a handful of grated Parmesan and the rest of the ragù and toss well.
Serve, passing round extra cheese for those who want it.