Combine 1/2 cup water, soy sauce, brown sugar and half of the sake in large ziploc bag and add the chicken thighs. Press out as much air as you can and seal the bag. Let this sit in the fridge for at least an hour.
To make the teriyaki sauce, just add the honey, soy sauce, the remaining sake to a small sauce pan and boil over medium heat until the sauce is glossy and slightly viscous (it won’t get quite as thick as the jarred types). It should take on a caramelized taste but be careful not to burn it.
When you’re ready to grill the chicken, turn the broiler on and move the oven rack up to the upper position. Place a baking sheet on the pan and keep the meat elevated off the pan by placing a wire rack on top (the rack of a toaster oven would do). Place the chicken thighs skin side down onto the rack.
Grill until brown then flip so the skin side faces up. Baste the skin side with teriyaki sauce and continue to broil until the skin is golden brown with just a few charred spots. Give the chicken one final baste with the teriyaki sauce and serve.