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1. Fill your Egg to the top of the firebox. Light the charcoal and bring the temp up to around 350-375 range. Once you have stabilized the Egg and there is no more smoke I put the 13x9 pan directly on the grate and put the raised grid on top of that to create an indirect cook.
2. While grill heats up, combine soy sauce, sugar, ginger, and garlic in small saucepan; stir together mirin and cornstarch in small bowl until no lumps remain, then stir mirin mixture into saucepan. Bring sauce to boil over medium-high heat, stirring occasionally; reduce heat to medium-low and simmer, stirring occasionally, until sauce is reduced to 3/4 cup and forms syrupy glaze, about 4 minutes. Cover to keep warm.
3. Sprinkle chicken on both sides with salt and pepper. Place chicken skin side up in single layer on your raised grid directly over the roasting pan. All 8 pieces should fit over the pan. Cover and cook until instant-read thermometer inserted into thickest part of thighs registers 170 degrees, 15 to 20 minutes. At this point I removed the chicken to a plate so I could remove the raised grid and drip pan. Then I put the chicken back on the grill over direct coals skin side down for 5 minutes to crisp up the skin.
4. Transfer chicken to cutting board; let rest 2 to 3 minutes. Cut meat crosswise into 1/2-inch-wide strips. Transfer chicken to serving platter; stir teriyaki sauce to recombine, then drizzle to taste over chicken. Serve immediately, passing remaining sauce separately. You can leave the skin on, but I removed it to make it more healthy. If you leave the skin on don't use the sauce until right before you eat so it doesn't get soggy. I served this over rice as there is plenty of sauce.