How to Make It
Combine all of the ingredients in a medium bowl and refrigerate.
In another bowl, cover the salt cod with water and soak overnight in the refrigerator, changing the water once or twice. Drain and rinse the fish and transfer to a large saucepan. Add the milk and simmer over moderate heat until the cod flakes easily, about 10 minutes. Drain the cod and discard the milk. Coarsely flake the fish and transfer it to a large bowl.
Meanwhile, in a medium saucepan, cover the potatoes with water. Bring to a boil and cook until just tender, about 10 minutes. Drain well and let cool slightly. Using your hands, mix the potatoes with the cod; the mixture should be chunky.
Heat the olive oil in a medium skillet. Add the scallions and cook over moderately low heat until tender, about 4 minutes. Add the scallions to the cod mixture along with the cream, tarragon and parsley. Season with salt and pepper. Shape the mixture into 3-inch cakes, about 3/4 inch thick. Lightly flour the cakes and set them on a baking sheet.
Heat 1/2 inch of vegetable oil in a large skillet. Working in batches, fry the cod cakes, turning once, until golden brown and crisp, 3 to 4 minutes. Drain on paper towels and keep warm while you cook the remaining cakes. Serve with the herbed mayonnaise.