Brine chicken. Add water to a large measuring cup. Add salt, sugar, coriander seed, fennel seed, and bay leaf. Whack garlic cloves with the side of a knife so it's partially smashed and cracked. Add to the brine. Stir together until the salt and sugar dissolves. Pour over the chicken drumsticks. Cover and brine for 24 hours or at minimum, overnight.
Make batter. In a large bowl, add the flour, corn starch (or potato starch), salt, curry powder, onion powder, and cold water. Whisk until well combined. The batter should be thin and runny. Remove the raw chicken from the brine and add directly into the batter. It's ok if a few coriander seeds and fennel seeds stick to the chicken but make sure to remove the garlic cloves and bay leaf. Mix until the chicken is well coated.
Deep fry chicken. Add 2-3 inches of avocado oil (or canola oil or vegetable oil) to a Dutch Oven. Heat over medium heat until the temperature reaches 350F/176C. Add the battered chicken to the hot oil, making sure not to crowd the pan. The drumsticks should float in the oil. Be careful -- the oil is hot! Deep fry until the chicken is golden brown, about 5-6 minutes. Remove from the hot oil and transfer to a cooling rack fitted over a sheet pan to catch the drips. Repeat until all the chicken is deep fried.
Deep fry chicken twice. After all the chicken has been deep fried the first time, deep fry for the second time. Heat the oil until it reaches 370F/187C on the thermometer. Add the fried chicken back to the hot oil and cook until dark golden brown, another 3-4 minutes.
TIP: *To ensure the chicken is completely cooked through, use a food thermometer. The internal temperature should reach 165C/74C.
Serve. Garnish with green onion, if desired. Enjoy with White Radish Kimchi (dongchimi), beer, and rice.