Is it wrong to say 巧克力是今天的甜点?
A 是B，B是A . No problem at all to my native ears.
Sound wrong. You need 今天 before the verb. 今天的 Adverbial phrase, 甜心 subject, 是 verb.
Always say the time in the beginning. That's why I would say your translation is wrong. I'm not a native speaker though.
In this sentence, 今天 is not really a "time point", it's just and adjective describing the noun 甜点
Better forget that "rule", IMHO...
Do you need the "de" in Chinese ? You don't need the possessive in English.
Today'S is possessive.
A "chocolate" what? The English feels like it ends missing a word, like "bar" or "cake" or "brownie" or "ice cream".
Just "chocolate", as in the uncountable noun.
Strange as it is is to have just chocolate as a dessert, we are not here to give duo menu suggestions; just to learn. From that standpoint it's not wrong to say chocolate as a noun here.
Why does it have to be any thing other than just chocolate? Just because you are used to having chocolate [something] for dessert, doesn't mean everyone has to. Rather closed-minded comment there.