Mix all the ingredients for teriyaki sauce in a large mixing bowl
Trim off excess fat from the chicken thighs. Place the chicken pieces on a cutting board and cover loosely with plastic wrap and use a meat mallet or pestle (I used pestle) to gently pound the meat into 1/2-inch thickness. Repeat with the rest of the chicken pieces
Preheat a large pan with oil. Place the chicken pieces on the pan in one layer and let them cook undisturbed so they can brown nicely, about 3 minutes and then flip over to the other sides and cook for another 3 minutes. Cook the chicken in two batches if necessary to avoid overcrowding the pan
Gently pour in the teriyaki sauce. It will sizzle. Let the chicken cook for another minute and then flip over. The sugar in the sauce is caramelizing and the sauce is getting thicker, lower the heat so you won't burn the sugar. Remove the chicken pieces into a clean cutting board and let them rest for 1 minute before slicing. Now, let the sauce cooks down to a shiny thick, but still somewhat a bit runny consistency. Remove from the heat and drizzle this sauce over the chicken pieces. Serve immediately