In a nutshell: 320 degrees Fahrenheit, 2 hours.
I received an email from someone asking how I cook kabocha squash, which is a hard winter squash, and sweet potatoes, particularly Japanese sweet potatoes and Garnet yams (which are really sweet potatoes but the name “yam” seems to have stuck). This is how I do it…
This is a photograph of my oven contents this morning:
That’s not a kabocha although it looks like one. It’s a buttercup. Both kabocha and buttercup squashes are dark green on the outside, bright orange on the inside, and sweet. The buttercup has a characteristic light green bulge on the bottom. It’s not real noticeable here but it can get pretty big. The buttercup’s flesh is moister than kabocha, but their tastes are similar. I’m extremely fortunate to have found some this year. Last year was a lean year for winter squashes.
As you can see, I wrap the potatoes, first in parchment paper then in aluminum foil. I’ve had sweet potatoes explode in the oven. What a mess! (Note – I also line the oven bottom for this reason.) If you don’t wrap them, the skin of the potato gets dry and crusty and the inside starts to boil. A potato bomb in the making. The parchment acts as a barrier between the aluminum and the potato. As the potato cooks, acidic steam escapes and slightly corrodes the aluminum which rubs off on the potato. No sense in eating more aluminum than I have to.
I have found, after cooking sweet potatoes for years, that no matter what kind of potato I cook (Japanese, Garnet, Jewel, Beauregard, white sweet potatoes, etc.) they all cook well at about 320 degrees Fahrenheit for 2 hours. A hotter oven doesn’t allow the potato to soften as much or to develop as much sweetness. At the end of two hours I turn the oven off, without opening it, and let them sit in there for another hour or two. Then I take them out, unwrap them, and let them cool on the counter for another hour of two. I don’t usually tell people that I take this long to cook them because they’ll think I’m crazy. I wouldn’t do it either if I didn’t get up at 4:30 a.m. One benefit of being a morning person. (I’m also usually cooking beans then too.)
The buttercup or kabocha squashes go into the same 320 degree oven as the potatoes to save time and oven heat. I don’t wrap the squashes. I’ve never had a problem with them exploding. Although they do weep, thus the cookie sheet. I don’t cut hard squashes before cooking them. Too much work and dangerous work at that. They go into the oven whole. After about 1.5 hours I check to see if they’re done. A moist squash like buttercup will have softened and start to leak. The kabocha won’t leak since it’s so dry (but that’s what makes it so good!) Time is hard to gage for squashes because they vary so much in size, but I usually remove them between 1 and 2 hours, not more. I put them, still whole, on a plate, cover them with foil (slows the cooling) and let them come to room temperature. It’s so easy to cut them afterwards! The skin is as soft as the flesh.
This buttercup squash took 2 hours to cook at 320 degrees F. Here’s a photo of it after it cooled. You can see how bright orange it is. The consistency is like mashed potatoes, very soft. The seeds easily spoon out. And sweet! It’s sweeter than a sweet potato. It’s a lot sweeter than a typical pumpkin, those round orange things sold to make pies. No wonder you have to use sugar to sweeten it. I think people don’t buy these dark green ones, even though they’re superior in taste, because they’re not very appealing on the outside. And because they’re so hard to deal with raw. Just cook it whole!
The long, slow cooking and long, slow cooling on the counter brings out the sweetness in both potatoes and hard squashes. They never taste the same if I try to hurry it. I’ve seen recipes that say to use a very hot oven, over 400 degrees, but it doesn’t produce the almost pudding-like consistency I can get by cooking slow and long.