"We do not have any cookies."
Translation:Nie mamy żadnych ciasteczek.
This would work if the word was masculine and the nominative for 'one cookie' was "jeden ciasteczek" ;) But it's neuter, and well, this is how it declines. Maybe someone else can explain why.
On a side note: I don't get why the course authors chose "ciasteczko" as a translation for "cookie", I would simply use "ciastko" as it's the main form, ciasteczko is a diminutive - little cookie...
Well, sponge biscuits are just a very special kind of biscuits. Anyway, in Britain 'biscuit' is the standard word for what Americans call a cookie.
By the way, 'biscuit' is originally a French word meaning "cooked/baked twice". And the German 'Zwieback' (which is something different) means the same. I just found out that this word is also used in English (and in Polish it's spelled 'cwibak').