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).
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
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?
You’re right, chocolate is a mass noun. To count chocolates, you can use «плитка шоколада» 'a bar of chocolate' or (colloquially) «шоколадка».