Mix Miso, Mirin, Sake, sugar, and soy sauce in a bowl. Marinade chicken thighs in Miso mixture for at least 3 hours, or overnight.

Preheat oven to 425F. Wipe marinade liquid well from chicken and place chicken on oiled aluminum foil spread over a sheet pan.

Bake for 15 minutes, turn over and bake 10-15 minutes more until cooked through.