In a large skillet, melt 2 tablespoons butter over medium heat. Add pancetta, onion, carrot, and celery and cook, stirring often, for 10 minutes or until lightly browned. Transfer to a heavy large saucepan.
In same skillet, heat olive oil. Cook ground beef and pork over medium heat, stirring to break up any lumps, until browned. Pour in the wine, increase the heat and boil briskly, stirring constantly, until almost all of the liquid has evaporated. Transfer ground meat mixture to the saucepan with the pancetta and vegetables. Set skillet aside.
Stir the beef stock and tomato paste into the saucepan. Bring sauce to a boil over high heat, then reduce the heat and simmer, partially covered, for 45 minutes, stirring occasionally.
Meanwhile, melt remaining 2 tablespoons of butter over medium-high heat in the original skillet. Add chicken livers and saute for 3 to 4 minutes, or until firm and lightly browned. Remove livers from skillet and dice. Set aside and add to sauce 10 minutes before it is done. A few minutes before serving, stir in the cream and let it heat through. Season sauce with nutmeg, salt, and pepper to taste.