Melt butter in a large pot or Dutch Oven over medium-high heat. Add onions, carrots, celery, garlic, tomato paste, and salt. Saute, stirring often, for 5 minutes or so. If the garlic or onion start to brown, lower the heat.
Add pancetta and continue to cook, stirring frequently, until the vegetables are fairly soft. Then, mix in the beef and pork about ⅓ at a time, letting most of the liquid evaporate before adding more.
Cook, stirring frequently, until all meat is incorporated and no longer pink. All the liquid should be evaporated, and brown bits should begin to stick to the bottom of the pot.
At this point, add in white wine to deglaze the pot. Scrape up the brown bits from the bottom of the pan, then lower the heat to prevent them from sticking again. Mix in milk, broth, tomatoes, salt, and pepper, and bring to a boil.
Once boiling, reduce heat to as low as possible to maintain a lazy simmer. Add bay leaves and parsley. Simmer on low, half-covered, for 4 hours, occasionally stirring.
If preparing pasta to go with the meat sauce, begin cooking pasta 10-12 minutes before your sauce is complete, according to package directions, in order to give yourself enough time for both to be ready at the same time.
When ready, remove bay leaves. If the sauce has not thickened to your liking yet, increase the heat slightly and simmer for another 15-30 minutes uncovered.
Serve bolognese meat sauce with pasta and plenty of freshly grated cheese! Enjoy!