In a large frying pan melt 2 Tablespoons of the butter over moderately low heat. Add onions and cook until onion is soft, about 5 minutes; add garlic and cook for 1 minutes longer. Transfer to a food processor or blender.
Melt remaining 4 Tablespoons butter in the same pan. Add livers and saute over medium-high heat, until browned outside but still rosy inside, 3-5 minutes. Sprinkle with salt, thyme, tarragon, pepper and Cognac. Cook, scraping up any brown bits from the bottom of the pan, for 1 minute. Scrape liver and pan juices into food processor. Puree until smooth. Scrape into a serving bowl or crock. Let cool to room temperature. Cover and refrigerate several hours or overnight.