I'm not an Italian Italian (like.. from Italy), but my family is of Italian background, so maybe this is relevant. Cakes pop up at birthdays, and every now and then for a change of pace, but yes the standards are biscuits and pastries. Any time you go to someone's house to visit, there'll be pastries and/or biscuits.
I'd say that "su" always means on. However, there are instances in which Italian and English disagree on using on or in. For example, Italians would say "il cibo è nel piatto" = the food in the plate, while in English one would say "the food is on the plate" (right? I am not an English native speaker). Bottom line, I don't think the problem is when su means on or in. The issue is learning when something is to be considered in or on something else. The sentence above (mettiamo lo zucchero sulle torte) does not have this problem though, since it means that we are putting sugar ON the cakes, on top. If you mean that you put sugar in the cakes (i.e., in the dough) then it would have been "mettiamo lo zucchero NELLE torte".
Don't know which country you're from (USA here) but I have never heard a native speaker say "in the plate".
A plate is FLAT. Something can only be IN something that has DEPTH. That would be a BOWL or some other type of container- but definitely not a PLATE. It's either "ON a PLATE" or "IN a BOWL".
I'm a native speaker in Australia, and we do use both "on" and "in". A friend compared it to saying that one is "in Germany", when really, physically speaking, one is on it :P .
I'll speculate that perhaps the "on" usage refers to the food being on top of something physically, whereas "in" refers to food being "in the domain of" the plate. So, food is "in" a plate in the same way that a person can be "in" a football team. This obviously does not mean that the person is some kind of chestburster alien who is physically situated inside the bodies of all the members of the football team. Indeed, even the food "in" a bowl is not really in the bowl in 3 dimensions, unless the bowl has a lid.
I am a native speaker from California, USA. I agree that nobody here would ever use 'in' because the plate is flat, BUT if you use the word 'plate' to mean the dish of food then it makes perfect sense... although it wouldn't be used in most conversation here. I imagine the person who baked the cake talking to someone else who bakes.
I also agree with the statement from above that it's not a question of 'su' meaning 'on' and 'in' but rather when 'on' or 'in' is used. Prepositions never translate 1-to-1 so these take some memorization.
I would say on the plate insyead of in. However just because something is flat does not mean it cannot contain something. In example take a country or state, we consider people are "in" these flat areas when they cross the boundry that designates it. It is not then a huge leap to consider things in a plate upon passing such a boundry.
I wished the lady speaking Italian would clearly say the singular "tortA" and clearly say the plural "tortE". Most of my mistakes are from not understanding what is said!!! I use another Duolingo app on my IPhone which has a male speaker clearly stating vowels when applicable. Buona fortuna!!!