I’m going to declare this a victory because while they may not be perfectly shaped, they taste really good.
I usually buy them in bakeries or convenience stores, where they’re “スイートポテト” (literally “sweet potato” in Japanese) but they’re a sweetened, baked pastry. Sweet potatoes (satsumaimo) are so cheap here that I decided to try and make some. I found a recipe online that did the trick nicely.
I think I’ll mash them more thoroughly next time, and take better care with presentation, but they definitely taste good!