Put liver, pepper, salt, and bay leaves in saucepan. Add enough water to cover. Simmer about one hour (at least 1/2 cup of water must be left in pot). Remove from heat and let get cool for at least 30 minutes. Strain (retaining liquid) and puree liver in Cuisinart or blender. Make sure it is very fine. Return to saucepan and add rosemary, remaining pot liquid and some of the butter. Heat on very low heat. Mix well and taste. Add more salt, pepper, and/or rosemary to taste. Put into one large bowl, or several small bowls. Cover with foil or plastic wrap, and refrigerate (or freeze).