Pat the chicken thighs as dry as possible, trim. To debone, place a chicken thigh on a cutting board, skin side facing down. Use the tip of a sharp knife to carefully scrape against the length of the bone, separating the meat from the bone. Work all the way around the bone until it's completely detached. Discard the bone & slice the thigh into 6 small cubes. Repeat with the remaining thighs. Set aside.
To a large bowl, add all listed dry ingredients. Whisk to combine well. Add the eggs & shoyu, then continue to whisk until the mixture forms a thick, smooth batter. Add the chicken pieces to the batter, stirring to coat well. Cover & transfer to the refrigerator to rest for at least 12 hours or up to 3 days.
About 30 minutes before you'd like to start frying your mochiko chicken, remove the marinated chicken from the refrigerator & set it on the counter to allow the chicken to come up to room temperature. Pour enough frying oil in a heavy-bottomed pot or deep skillet (I use a 4-quart Dutch oven) such that the oil is about 1 inch deep. Preheat over medium-high heat. The oil is ready once it sizzles when you sprinkle a little flour into the skillet, approx. 350-375 degrees F.
Once oil is hot, transfer chicken into the hot oil, working in batches. Fry 4-5 minutes, turning the pieces about halfway through to promote even browning, until the mochiko chicken is crispy, golden brown, & an instant read thermometer inserted in the thickest portion of each piece registers an internal temperature of 165 degrees F. Use a spider strainer to transfer the chicken to a paper towel-lined plate to drain. Season with furikake while the chicken is still warm. Repeat with remaining chicken.