In a large skillet, cook the mustard seeds over moderately high heat until toasted, about 1 minute. Transfer to a plate. Heat 4 tablespoons of the butter in the skillet. Add the shallots and cook over moderate heat until softened. Add the livers, season with salt and pepper and cook until barely pink inside, about 8 minutes. Add the cognac and simmer for 1 minute. Scrape into a food processor; let cool.