I have tried tempura and it is close. The one I had in the forgotten restaurant was not tempura however. They used it on shrimp and onion rings. The batter at Fish Daddy's is close.
Soak shrimp in regular whole milk (not buttermilk) for a minute or two
Remove shrimp from milk and coat in a self-rising flour (or you can use a mixture of all purpose flour and baking soda) and your preference of seasoning
Shake off excess batter and drop into a high smoke point oil at a temp in the 360-380 range. Don't crowd the fryer
Pull and rest on paper towels for a few minutes and then eat
I like to add cayenne to the milk mixture
The combo of dry batter, milk instead of butter milk/egg wash, and the self rising flour will make for a light batter and let the taste of the seafood shine through
They key to a thin/light batter is having a fairly thin egg/milk/seasoning mixture and using seasoned self-rising flour. Sift the flour prior to use as well.
Dip in egg wash and dredge in flour mix and straight into the hot oil. Thicker the egg/milk mix, the more flour sticks and thus makes a heavier batter.
Honestly, the ingredients are awesome and hard to screw up. I've never once measured. If I had to guess, I would use about two cups of sweet thai chile sauce, juice of one lime and quarter cup fresh cilantro. I could be way off though. Just do it to taste.