I'm guessing Tofu is a relatively young girl? Usually younger hamsters just wee as and when it happens. It's not uncommon for them to do it in their house during the early days. At least this used to be the case with my girl. I have two separate sand baths for her, since dwarf hamsters love bathing in sand. The other one, which she uses as a toilet is placed in a corner of the cage. I'm not entirely sure if two are needed, but my girl clearly prefers it.
The best way to toilet train a hamster if you cannot seem to find a preferred pee corner is to basically just scoop the substrate that she has peed on and place it into the litter tray. Eventually, she'll figure out what it means and start using it as the toilet. It could take just a day or a week or two. Until then, you just got to be patient and train her such. Young hamsters are very much like human kids. They slowly learn how to hold and use the toilet properly when taught.
Here's a video by ErinsAnimals for litter training Hamsters:
https://www.youtube.com/watch?v=c0h9HQB_ofs