As Gazelle1596 said, you can't say sufuriani ya.
The -ni suffix converts the noun into one of the three locative classes. For these, the equivalent words for of are pa (class 16, specific location), kwa (class 17, generic or approximate location), mwa (class 18, internal location).
When talking about entering the "cake tin" (if sufuria is the right word) you'd say sufuriani mwa keki because you're talking about the inside of the tin.
However, in practice, mwa is pretty rare and when you're further describing a noun with a following phrase, it's more common to precede it with kwenya or katika rather than add -ni, so you could simply say kwenya/katika sufuria ya.