Whisk together all buttermilk marinade ingredients in a mixing bowl. Marinate chicken breasts for at least 3 hours or up to 1 day under refrigeration.
Make spiced hoisin
Mix all ingredients and refrigerate until ready to use. The sauce can be kept in an airtight container in the fridge for up to 2 weeks.
Fry chicken
Heat the oil on medium-high heat to the desired temperature, usually around 350F (175C).
Pick one piece and dredge the marinated chicken from the buttermilk mixture into the flour mixture. Then, dip that piece again back in the buttermilk. Finally, dredge again in the flour.
Carefully add dredged chicken breast to the hot oil.
Deep fry the chicken breast until done, approximately 7-10 minutes. The outer layer of the fried chicken breast should be crispy and golden brown.
Remove the deep-fried chicken breast from the oil and hold it over the pan, allowing the excess fat to drain.
Transfer the chicken to a pan or a plate lined with absorbent paper. Season lightly with salt and pepper.
Assemble
Cut burger buns in half. On a clean surface or a clean plate, spread a thin layer of spiced hoisin on both halves of the buns.
Then add basil to one half, top it with fried crispy chicken (drizzle more spice hoisin on top of the chicken if desired), culantro, a slice of tomato, and cilantro, and then the other half of the bun.
Serve immediately with French fried on the side.
Notes
If the deep-fried chicken is not used immediately, place it in the oven at 250F to keep it warm and crispy or under a heat lamp.
Substitutions
The buttermilk in this recipe can be replaced with 1 cup of whole milk, ½ cup heavy cream, and 1 tsp fresh squeezed lemon/lime juice.
For a regular fried chicken sandwich, replace the spiced hoisin sauce and Asian herbs with mayo spread and lettuce.