For the tempura flour:
Combine all-purpose flour, corn starch, turmeric powder, baking powder, 1/4 teaspoon of sugar, red chili powder and 2 teaspoons of salt. Sift all the dry ingredients.
For the tempura batter:
Divide the seltzer water into 2 measuring cups: one with 2-1/3 cups and another with 1/3 cup.
Add 2-1/3 cups of seltzer water and crushed ice to the tempura flour until incorporated. Do NOT over-mix; it's okay if the batter is still lumpy.
In another bowl, dissolve the rice flour into 1/3 cup of seltzer water.
Combine the two mixtures and stir to even them up. Once more, do NOT over-mix in order to get an airy result! The consistency of the tempura batter should be similar to pancake batter. Add the remaining seltzer water if necessary (I didn't).
Allow to rest for about an hour.
For the shrimp:
In a bowl, combine 1-1/2 teaspoons of salt, 3/4 teaspoon of sugar, 1/2 teaspoon of cayenne powder and pepper.
Use fresh, good looking shrimp. If using frozen shrimp, it's not necessary to thaw them. Remove and discard the head of the shrimp if it's still attached. Carefully shell and de-vein the black part of the shrimp using a sharp hook-like paring knife. Make sure to leave the tip of the tailon, if possible. Rinse the shrimp under cold running water and pat dry using a paper towel. There should be as little water as possible. Repeat the same procedure for each shrimp. This step is tedious but essential for a good result.
Spread the shrimp on a large tray. Generously coat the shrimp with garlic. Sprinkle the shrimp with the spices from the seasoning bowl. Add about 2 teaspoons of oil. Toss well. Plastic wrap and let the shrimp marinate for 15-20 minutes in the refrigerator.
For the sweet potatoes:
Peel and cut the sweet potatoes into 1 to 1-1/2-inch matchsticks (to your liking). Place them in the tempura batter. Add 2 teaspoons of oil.
Layer a cooling rack, previously lined with paper towels if you like, on top of a baking sheet (for easy clean-up of the drained oil).
In a large Dutch oven (or any regular deep-fryer), heat the oil for about 2 minutes over high heat. There should be at least a 2-inch-high level of oil. Wait until the oil is slightly bubbly (not too hot).
Test the oil by dropping a teaspoon of tempura batter into the hot oil. It should float but not swell.
Using a spatula, gather 3 sweet potato match-sticks. Dip a shrimp into the tempura batter to coat completely. Allow the excess batter to drip back into the tempura batter bowl. Place a shrimp on top of the 3 match-sticks. Drizzle an additional teaspoon of tempura batter over the shrimp to ensure that it adheres well to the sweet potatoes and place in the hot oil.
Fry in batches. Make sure the shrimp fritters don't touch each other. Lower the heat to medium. Deep fry for 2-3 minutes per batch. The fritters will start to pop and increase in volume. Use a splatter screen to prevent the oil from jumping everywhere . Flip each piece using a spider skimmer and cook for about a minute until golden on all sides. Remove each fritter, draining as much oil as possible and transfer them on the cooling rack. Continue with the remaining shrimp.
Serve immediately. You can eat the shrimp fritter simply as is or wrapped in lettuce filled with Thai basil, cilantro and Vietnamese mint.
I used seltzer water. You can also use club soda but in that case, do not add more salt. Another option is to use beer, which was Maman's (my mom's) option.
Sifting dry ingredients helps get rid of nasty lumps of flour and aerates the mixture when liquid is added.
I only add a small amount of turmeric powder so as not to alter the flavor of the batter. It's used as a natural food coloring to make the shrimp fritters look more vibrant.
The traditional way to eat these shrimp fritters is by rolling them in a lettuce leaf as a salad roll and dipping them in fish sauce (nước mắm chấm).
You can buy rice flour at the store but I prefer grinding my own though (use jasmine rice). That way I know all the nutrients are preserved. I use the Nutrimill brand mill. I got it several months ago and it's very useful to make any kind of flour. It's a great help for people with gluten allergies as well.
For optimum results when heating the frying oil, the thermometer should register 350°F to 375°F. Heat the oil over medium to high heat.