Mix flour, sugar, salt, and yeast in a bowl.
Combine milk, warm water, and butter, then mix into dry ingredients to form a dough.
Knead for 10 minutes, then let rise until doubled (about 1–2 hours).
Marinate chicken in buttermilk, salt, and spices for at least 1 hour.
Shape dough into small ovals, brush with oil, fold, and let rise again for 1 hour.
Heat oil and prepare crispy coating mixture.
Coat chicken pieces and deep fry until golden and cooked through (3–5 minutes).
Steam bao buns for 10 minutes until fluffy.
Cook sauce ingredients in a pan until thickened.
Toss fried chicken in the sauce.
Fill bao buns with chicken and top with onion, cucumber, coriander, and sesame seeds.
This Korean Chicken Bao is best served fresh and warm.