Dice back-fat, liver and meats and mix in a bowl with brandy, truffle purée, leek, salt and dried herbs, cover and refrigerate overnight.
Preheat oven to 160C. Soak the caul fat in cold water for 30 minutes, then rinse and squeeze out excess water, ensuring fat is dry. Line a 1.2-litre ceramic or cast-aluminium terrine mould with caul fat, leaving enough overhanging to cover terrine.
Coarsely mince marinated meats, including any liquid (if you don’t have a mincer, finely chop the meats), then mix in an electric mixer on low speed to combine well. Press firmly into terrine mould, pushing into corners and sides. Fold overhanging caul fat over to cover meat, top with bay leaves, cover tightly with foil and place in a deep roasting pan. Fill the pan with lukewarm water to three-quarters of the way up the sides of the mould and bake until internal temperature reaches 65C on a meat thermometer (about 1 hour 15 minutes). Cool to room temperature (3-4 hours; to reduce this time, place the mould in a bath of cold water when it’s no longer steaming), cover, then refrigerate until chilled and firm (overnight). Terrine can be made 2 days ahead.
Turn out terrine, scrape off excess jelly, cut into thick slices and serve at room temperature with cornichons, butter lettuce, chopped chives, and crusty baguette.
