Marinate the chicken: In a small bowl, add sour cream, Worcestershire sauce, vinegar, ground black pepper, and salt. Toss the chicken in and mix to coat, and then chill for at least 2 hours or overnight.
Coat the chicken: In a small bowl, beat the egg using a fork. In another small bowl, mix flour, cornstarch and garlic powder. Now, dip the chicken in the egg, then in the flour mixture, making sure they are well coated.
Frying the chicken: Heat the oil in a large frying pan over medium heat. Once hot, put the chicken thighs into the hot oil and fry for 5-6 minutes on each side or until golden brown. Remove from the heat, drain off the excess oil on a paper towel.
Assemble the burger: Cut the bun open and spread butter on the bottom half. Add lettuce, top it with a chicken thigh, add 2 slices of pickled cucumber, and add some grated carrots. Drizzle with sriracha sauce, and finish with the other half of the bun.
Notes
You may store leftovers in the fridge for up to 4 days or in the freezer for up to 3 months.