(A 6-10 pound fish is ideal for this collar, big enough for 2-4 people to share.)
Mix all dry ingredients (rice flour, tapioca starch, baking powder, Korean chili flake, garlic powder, onion powder, and paprika) together in a large bowl and whisk to mix, then add soda water until it has a pancake batter consistency. Reserve leftover soda water in case the batter thickens after sitting for a while.
In a small sauce pot, add heavy cream and chilis, then reduce mixture over medium heat until it starts to thicken. Reduce the amount of heat to the pot so that you will be able to emulsify the butter into the cream and chili reduction. If it is too hot or cold, the butter will break. (I always check the temperature of the bottom of the pot by actually feeling the temperature with my finger. It should be hot to the touch but not burn your finger.)
Mount butter slowly by adding only 1 or 2 pieces at a time and whisking them completely in. When all butter is whisked in, season with salt and pepper to taste and take butter sauce off of heat completely but keep warm. Add the juice of two limes to the butter sauce to brighten it up and keep sauce warm.
Remove the skin and scales from the fish collar and separate the collar from the head and body of the fish. Season liberally with salt/pepper and coat with the rice flour. Dip into rice flour batter and directly into a deep fryer at 375 degrees, then cook for about 6-8 minutes or until the interior of the collar reaches 130 degrees. Drain off excess oil and season with salt/pepper again.
To plate, place seasoned collar on a large plate and top with hand-torn herbs. Spoon butter sauce over top of the herbs and collars and serve immediately.