Most instructions on the web suggest roasting kabocha at about 400° F. My usual protocol involves baking the whole squash for about 20 minutes at 400 °F, scooping out the seeds, cutting into 24-32 wedges (depending on size), then roasting for 40 more minutes. At this time of year, I’ve found a regular bounty of kabocha with the ideal, somewhat dry egg-like texture, but some have clearly been sweeter than others.
A common technique to increase sweetness of sweet potatoes (including Japanese sweet potatoes) is to bake them at a lower temperature for a longer period of time, giving the starches a greater opportunity to break down. I wondered if the same rules could be applied to kabocha.
With my most recent sqaush, I roasted some wedges at 325 °F for 80 minutes and some at 400° F for 40 minutes (the batch on the left of the image is the higher temperature). The 400 °F wedges were clearly darker with greater caramelization. To my surprise, if anything the higher temperature yielded sweeter squash. It’s possible that the lower temperature specimens simply needed more cooking, but for now, I’d suggest sticking to 400 °F.
Update: I did another test, this time cooking part of the squash at 400 °F for 45 minutes and part at 250 °F for 150 minutes. Both versions were well cooked and dry, but the texture and flavor was definitely superior in with the higher temperature. The lower temperature squash was grassier and less sweet compared to the higher temperature. Stick with 400 °F.