For the kipper fishcakes, place the kippers and milk into a saucepan and bring to the boil. Remove from the heat and set aside to cool. Once cool, drain and flake the fish, removing as many bones as possible, and set aside. (Discard the milk.)
Mash the cooked potatoes in a large bowl until smooth, then add the parsley, egg yolk and kippers. Season, to taste, with salt and freshly ground black pepper. Stir until well combined.
Shape the mixture into four equal-sized patties, then chill in the fridge for two hours.
Sprinkle the flour onto a plate. Beat the eggs in a bowl. Sprinkle the Japanese panko breadcrumbs onto a separate plate. Once the fishcakes have chilled, dredge each in the flour, then dip in the egg and finally roll in the breadcrumbs.
Heat the olive oil in a frying pan over a medium heat and fry the fishcakes for three minutes on each side, or until golden-brown on each side. Keep warm in the oven.
For the tartare sauce, blend the egg yolks, white wine vinegar, Dijon mustard and salt in a food processor. While the motor is running, gradually pour the oil into the food processor in a steady stream until the mixture thickens. Stir in the cornichons, capers, parsley, chives and lemon juice. Season, to taste, with salt and freshly ground black pepper.
For the watercress salad, place the watercress in a bowl, add the oil and season, to taste, with salt and freshly ground black pepper. Mix together until well combined.
To serve, spoon the tartare sauce into a four small bowls and place one onto each of four serving plates, along with a kipper fishcake. Divide the watercress salad among the plates and garnish with the lemon wedges.