Marinate the chicken about 24 hours before cooking. Put the miso and mirin in a medium bowl. Whisk until the miso is creamy. Add in the sake and sugar. Mix well.
Place the chicken into a storage bag or shallow container that will allow the chicken to fit in a single layer. Pour the marinade over the chicken and place in the refrigerator for 24 hours.
After the chicken has marinated for 24 hours, turn on the oven broiler function to high. Line a baking sheet with foil and put a broiler rack on top of it. Remove chicken from marinade and pat lightly with paper towels before arranging chicken on the broiler rack. Discard marinade.
Broil chicken for about 12-14 minutes, turning about ¾ through the cooking time. The chicken should have nice browning and some char.
Remove from oven and let rest for about 5-10 minutes before slicing. Serve immediately.