Arrange the chicken thighs on a work surface, smooth side up. Using kitchen shears, make a few cuts in the thickest part of the chicken thigh. This will help the chicken cook evenly and will also help the brine absorb. Add the chicken to a large bowl and cover with 2 cups of pickle juice. Cover with plastic wrap and let marinate in the fridge for 2 hours.
After 2 hours, remove the chicken from the pickle juice and pat dry. Let the chicken sit at room temperature for 20 to 30 minutes.
Fill a large Dutch oven with about 3-inches of vegetable, attach a deep fat fry thermometer and heat to 350 degrees F.
When the oil is almost heated, make your batter: Add the tempura mix, tapioca flour, 1 teaspoon salt and 1 teaspoon pepper to a medium bowl, pour in the club soda and whisk to combine. It should be slightly thinner than pancake batter.
Working in batches, dredge the chicken in the batter, allowing the batter to cover the chicken fully. Shake off any excess batter. Transfer the chicken to the hot oil and fry until golden and crisp with an internal temperature is 165 degrees F, 8 to 10 minutes. Remove the chicken to a wire rack and sprinkle with salt. Fry the remaining chicken.
To assemble the sandwiches: open the buns and lay them cut side-up on a clean work surface. Spread the mayonnaise evenly over the top and bottom halves of the buns. Place a piece of chicken on each bottom bun. Top each piece of chicken with a heaping 1/2 cup of slaw. Place the top buns on the sandwiches and serve.
Yield:About 4 cups
Add the shredded cabbage to a medium bowl and add the pickle juice, vegetable oil and 1/2 teaspoon salt. Using clean hands, massage the cabbage to help it slightly break down. Add the mustard, pickle, onion and dill. Toss well to combine. Keep the slaw refrigerated until you’re ready to assemble the sandwiches. Any leftover slaw can be stored in an airtight container for 3 to 5 days.