How to Make It
In a food processor, pulse the hake until minced. Add the baking powder, 2 teaspoons of salt and 1/4 teaspoon of black pepper and pulse until combined. With the machine on, add the cream and puree until smooth and light in texture, about 2 minutes.
In a large nonstick skillet, melt 2 tablespoons of the butter. Spoon 1/4 cup of the fish mixture into the skillet and flatten to a 1/2-inch-thick cake. Repeat 3 more times to make 4 cakes. Cook over moderately low heat, turning once, until golden and cooked through, 4 to 5 minutes. Transfer the fish cakes to a platter and keep warm. Repeat with the remaining butter and fish mixture to make 8 more cakes.
Meanwhile, in a small bowl, stir the mayonnaise with the 1/4 cup of dill and the vinegar and season with salt and white pepper. Serve the fish cakes with the dill mayonnaise and a sprinkling of chopped dill.