It isn't because I believe шоколад is in the Accusative case here and because it is a Masculine inanimate noun it takes the nominative form (no change).
Шоколаду is the partitive case, it equates to saying "buy some chocolate". Not much difference in meaning, they can generally be used interchangeably.
Is «шокола́ду» really a standard form? It feels dialectal and non-standard to me. I'd use «шокола́да».
I might be hyper-correcting, though (Belarusian šakaładu and Ukrainian шоколаду are normal Genitive forms in these languages, so I might be avoiding these in Russian because I know they're different in Belarusian and Ukrainian).
Купи шоколада, купи шоколаду - это старомодно, но применимо. Сейчас говорят - купи шоколадку, купи шоколад.
I am fairly certain I have seen "шоколаду", and see http://starling.rinet.ru/cgi-bin/morph.cgi?flags=endnnnnp&root=config&word=%F8%EE%EA%EE%EB%E0%E4%F3
Partitive when there is a separate form is labeled as "Genitive 2" (compare http://starling.rinet.ru/cgi-bin/morph.cgi?flags=endnnnnp&root=config&word=%F7%E0%FE
I know for some words that have a separate partitive form, it's not used instead of genitive consistently, e.g. "сахару" is an example I've seen of a partitive form that is used only inconsistently. It may well be that "шоколад" is one of these words and the genitive is used at least as commonly as the partitive
Because шокола́д is a masculine noun, and the accusative form is the same as the nominative.
At the point in the course where this sentence appears, only through a few common examples but not "in general" in a lesson.
Like "Дайте", "Скажите", "здравствуйте" (and their corresponding forms for "ты": "Дай", "Скажи", "здравствуй".
In most cases, I've seen that perfective verbs are formed from imperfective verbs by adding some kind of prefix, like "у", "при", "на", "по" and such. However, it seems that "покупа́ть" and "купить" are a special exception to this pattern: The imperfective verb is the one using de extra prefix "по". Is there a reason? Is this a very rare example or is it common for this "pattern" to be broken like this?
Is this how one would answer a question like "What kind of ice cream do you want?"
In that context you would use an adjective which would agree in gender with "ice cream" - какое мороженое? Шоколадное. Usual non-native speaker warning but this agrees with what I found from searching too.
Why is the verb form to buy spelt "купи" ? Sorry for simple qustion but it doesn't match any verb endings i've learned and I'm a beginner ! :)
I believe this is an imperative form; and the adjectival form of "spell" is "spelled". Spelt is a grain :).
I believe "chocolate" is considered a mass noun in Russian; and thus the implied request is to buy "some chocolate". "A chocolate" would mean a single truffle or some such; but I'm not sure how this would be expressed if that is what the speaker meant.
You’re right, chocolate is a mass noun. To count chocolates, you can use «плитка шоколада» 'a bar of chocolate' or (colloquially) «шоколадка».