Follow these steps to make the perfect spicy crispy chicken burger:

Step 1: Marinate Chicken

1. In a bowl, whisk together the egg whites, flour, gochugaru, paprika, garlic powder, salt, and sugar.
2. Coat the chicken thighs thoroughly in this mixture.
3. Marinate the chicken for at least 3 hours or preferably overnight for maximum flavor.

Step 2: Prepare Coating

1. In a separate bowl, combine all the coating ingredients: flour, gochugaru, paprika, garlic powder, baking powder, salt, and sugar.

Step 3: Make Sauce

1. Mix all sauce ingredients in a bowl: Japanese mayonnaise, sweet chili sauce, hot sauce, soy sauce, vinegar, and chili flakes.

Step 4: Dredging Process

1. Dredge a marinated chicken thigh in the coating mix.
2. Dip the coated chicken in ice-cold water for 5 seconds, then let it sit for 10 seconds.
3. Coat the chicken again in the dry mix.
4. Repeat for all chicken pieces and let them rest for 5 minutes to ensure the coating sticks well.

Step 5: Fry Chicken

1. Heat oil in a deep pan to medium heat.
2. Fry the chicken for 10-15 minutes until it achieves a golden brown color.
3. Remove from oil and let rest, sprinkling with additional salt and gochugaru if desired.

Step 6: Assemble Burger

1. Toast the burger buns and spread the sauce on both halves.
2. Layer with crispy chicken, cheese, and lettuce.
3. Top with the other bun and your spicy crispy chicken burger is ready to be enjoyed!