If storing for several days, top pate with a layer of melted butter to prevent discoloration.
Butter the inside of a small crock; set aside. Heat the olive oil in a large skillet over medium-low heat. Add the onion, and cook, stirring frequently, until translucent, 8 to 10 minutes. Reduce heat to low; add livers, and season with salt and pepper. Cover and cook, stirring frequently, until just cooked through, about 15 minutes. Lift livers from pan; transfer to a bowl until completely cool.
Return pan to medium heat; add cognac. Tilt pan to ignite cognac; cook until most of the liquid has evaporated, about 4 minutes. Remove from heat; set aside until cool.
In the bowl of a food processor, combine the reserved livers and the pan juices. Chop the truffles, and add them and the butter to food processor. Process until almost completely smooth. Season with salt and pepper. Transfer pate to prepared crock, smooth the top, cover well, and refrigerate up to 3 days, until ready to serve.