In a small microwave-safe bowl, combine the sauce ingredients: 2 Tbsp soy sauce, 2 Tbsp mirin, 2 Tbsp sake, and 1 Tbsp sugar. Microwave the sauce until hot. Mix well to dissolve the sugar and set aside.

Place ¾–1 lb boneless, skinless chicken thighs smooth side down on a cutting board. Use the scoring method to create an even thickness so the thighs cook evenly. Cut slits across the meat every ½ inch (1.3 cm) without slicing all the way through. Make deeper slits on the chicken's thick parts and skip scoring the thin parts. After each cut, press open and flatten the meat with your fingers.

Lightly season the chicken pieces with ⅛ tsp Diamond Crystal kosher salt and ⅛ tsp freshly ground black pepper.

Heat a large frying pan over medium to medium-high heat. When the pan is hot, add 1 Tbsp neutral oil. When the oil is hot, place the chicken smooth side down in the pan. Cook the chicken for 5 minutes, pressing down hard on the chicken with a spatula for 5–10 seconds to give the presentation side a nice sear.

Flip over the chicken. Immediately press it down again with the spatula for 5–10 seconds. Switch to low heat, then cover with a lid and steam the chicken for 3 minutes.

Open the lid. Use a paper towel to wipe the excess grease and protein from the pan. Increase the stove's heat to medium and add the teriyaki sauce to the pan. Tilt the pan and frequently spoon the sauce over the chicken.

When the chicken is well coated with sauce and cooked through, transfer it to a cutting board. Continue cooking the sauce until it's thick enough to your liking. Transfer it to a bowl.

Slice the chicken into bite-sized pieces. Transfer them to a ceramic plate. Sear with a kitchen butane torch to enhance the flavor. Serve on a plate with the sauce drizzled on top.