In a large mixing bowl, combine all-purpose flour, caster sugar, salt, and instant dried yeast. In a separate jug, mix warm milk, warm water, and melted unsalted butter until combined. Pour the wet ingredients into the dry ingredients and knead the dough for about 10 minutes until smooth and elastic. Meanwhile, marinate chicken chunks in buttermilk and seasoning, letting them rest for at least 1 hour, preferably overnight, for maximum flavor.

Once the dough has risen in an oiled bowl for 90 minutes to 2 hours until doubled in size, gently punch it down. Knead the dough again and divide it into 20 equal balls. Shape each ball into an oval, brushing them lightly with olive oil. Fold each oval in half and place on parchment squares. Let them rise again for about 1 hour until puffy and increased in size.

In a deep skillet, heat vegetable oil to 350°F (175°C) for frying. While the oil is heating, coat the marinated chicken pieces in the crispy coating mix of flour and spices, ensuring each piece is completely covered. Carefully deep fry the chicken in batches for about 3-5 minutes, turning until golden brown and crispy. In a saucepan, combine gochujang, sesame oil, honey, brown sugar, soy sauce, garlic, and ginger; simmer on low heat until the glaze thickens.

Prepare a steamer setup by bringing water to a boil in a pot. Place the shaped bao buns in the steamer basket, ensuring they have space to expand. Steam the buns in batches for about 10 minutes each until fluffy and fully cooked.

Once the bao buns are steamed, take them out and let them cool slightly. Stuff each bao bun generously with the crispy fried chicken pieces and top with your choice of green onions, lettuce, Korean spicy mayo, sesame seeds, and pickled radishes. Serve warm.