Drain the livers, reserving the marinade. Pat them dry with paper towels. In a large heavy skillet, heat the 4 tablespoons butter over medium-high heat. Add the livers, the reserved bay leaves, and garlic, and toss 20 seconds or until the livers lose their red color. Pour in the marinade and boil about 2 minutes. The livers should be firm but still pink inside. Scoop them out of the pan with a slotted spoon and set aside. Continue boiling down the liquid until all of it has evaporated and the butter sputters. Add the 2 tablespoons of stock to the pan and scrape the contents over the livers. Cool to room temperature. Puree the livers in a food processor until smooth. Season aggressively with salt and freshly ground pepper.
Have a large bowl of ice handy. In a medium bowl, whip the 10 tablespoons butter with a portable beater at medium speed until fluffy. Gradually beat in the liver puree, then set the bowl over ice and continue beating at medium speed 8 to 10 minutes, or until the mixture has lightened in color and is very fluffy. Cover and refrigerate 1 to 2 hours.
To serve, bring the mousse close to room temperature. Spoon it into individual 3-inch ramekins or crocks, or mound it in a 3-cup terrine. Sprinkle generously with the shaved truffle. Accompany with bite-sized pieces of crusty bread.
Yield: 1 1/2 quarts