In a small bowl, stir together all the sauce ingredients until smooth and combined. Cover with plastic wrap and refrigerate until ready to use.
Place about 1 ½-2 inches of oil in a cast iron pan over medium heat, and heat it to 375 degrees F.
In the meantime, combine shrimp and buttermilk in a medium bowl.
Add cornstarch to a shallow bowl.
Remove shrimp from buttermilk, toss one by one (or just a few at a time) in cornstarch to coat well. Remove shrimp from cornstarch and shake gently any excess.
Add shrimp to the hot oil and fry it in batches. Only cook for about 1 minute per side, until golden and crispy.
Place cooked shrimp on a paper towel-lined plate and cook remaining shrimp.
Once done frying, add all the shrimp to a large bowl and pour ¾ of the sauce on top. Toss gently to coat evenly.
Sprinkle with green onions if desired and serve with rice with remaining sauce on the side for dipping.
