Place soy sauce, sugars, ginger, mirin, gochujang, sesame oil and rice vinegar in a bowl and whisk until well combined.
Taste and adjust seasonings to your liking.
Sear the chicken thigs on high heat in a wok or medium to large sized skillet.
Add in chopped garlic, red pepper powder and stir.
Once the chicken thighs are almost cooked through, remove from the heat and set aside.
Now, pour the spicy teriyaki sauce into the hot wok and stir occasionally until sauce bubbles and thickens.
As sauce begins to thicken, place par cooked chicken thighs back into the wok with the sauce.
Finally, simmer the chicken thighs and sauce until cooked through and sauce is slightly thickened.