Prepare the char siu chicken: Pat the chicken thighs with paper towels to remove excess moisture, then trim and discard any big fat pads. If the thighs are large or super uneven in thickness, butterfly each one. Lay the thigh, smooth-side down, on your
cutting board. Wielding your
knife horizontally, slash the big mound of flesh to create a flap of meat, stopping just shy of cutting all the way through. Fold back the meat flap that you just created. The thigh should now be about 50% longer and relatively even in thickness. If the result seems awkwardly large, cut it crosswise into two smaller, square-ish pieces. Set aside.