Make these 3–4 hours ahead. Put the chopped chicken, anchovies, capers, tartare sauce and mayonnaise in a bowl, season to taste with black pepper and mix well together. Trim and shred the lettuce. Spread each slice of bread thinly with butter, then spread the chicken mixture evenly over each brown slice. Sprinkle the shredded lettuce on the chicken and top with the white slices. Stack the sandwiches together, wrap in plastic wrap and chill for 3–4 hours. Just before serving, unwrap, trim off the crusts, cut each sandwich into four triangles and arrange on a serving dish.