I was taught that a singular cookie was "Ciasteczka", not "Ciasteczko"...
I'm assuming "kanapki" is plural of a sandwich, so what is the singular form?
There is a post about creating plurals in Polish: https://www.duolingo.com/comment/12354055
You can also check most popular types of declension - there are also plural nouns:
Here's the chart with endings. Unfortunately, there are many endigs for every case, so you would need to check dictionary to be sure. A Polish person with decent vocabulary would be able to decline many new words oneself, but this requires some experience.
Yes, it's neuter. So is "ciastko" which is I think rather more common word.
"Ciastko" would be more correct. "Ciasteczko" is a cute diminutive we use with children and family. I suspect though it might be easier to pronounce to an English speaker.
I agree personally, but the course creators went with "ciasteczko" as the default. We cannot change it.
They must eat a lot of cookies in Poland, thats all we ever seem to learn. Maybe the odd apple or sandwich