Whisk all the marinade ingredients together in a bowl until the sugar and salt dissolve. Put the chicken pieces in a bowl, add the marinade and toss to coat. Cover with plastic wrap and leave to marinate for at least 4 hours. Overnight in the fridge is even better.
Mix everything together in a bowl until combined.
To make the kimchi mayo, purée the kimchi in a food processor until smooth. Transfer to a bowl, add the mayonnaise and whisk to combine. Set aside.
Fill a deep-fryer or large heavy-based saucepan one-third full with oil and heat to 170°C (325°F) or until a cube of bread dropped into the oil turns golden in 20 seconds. Remove the chicken pieces from the marinade and cover each piece in the coating mixture until it feels dry. Deep-fry in batches in the hot oil for 5–6 minutes, or until the skin is golden and crispy. Remove with a slotted spoon, drain on paper towels and sprinkle with spicy salt. Serve immediately with the kimchi mayo.