Combine the sake, mirin, soy sauce, sugar, ginger and sesame oil in a dish that you can steep the chicken in, (I use a 23cm square dish, but anything of similar dimensions would do).
Add the prepared chicken pieces and leave for 15 minutes.
Heat the peanut oil in a large shallow frying pan or casserole (that has a lid) over medium heat. Using a slotted spoon, transfer the chicken pieces from the marinade to the pan. Saute them until they look cooked on the outside.
Add the marinade to the chicken pieces and bring to a bubble, then turn down the heat to a gentle simmer, put the lid on and cook for about 5 minutes. Cut into a piece of chicken to make sure it's cooked through.
Remove the cooked pieces with a slotted spoon (you could keep them in a bowl covered in aluminum foil to keep them warm) and turn the heat up under the pan to let the liquid boil down to a thick dark syrup.
Return the chicken pieces back to the pan, stir well so that all the chicken pieces are coated in the sticky savory syrup. Serve with a comforting pile of sushi rice and perhaps some steamed baby bok choy or other veggies of your choice.