You have a few questions embedded in here, and I will try to address each one.
First, "kind" is singular. It can be pluralized by saying "kinds." It can be used in either its singular or plural form, depending on what you want to say, just as any other simple "count noun" can. You can have one kind of something, or you can have many kinds of something. For example, there are "many kinds of lettuce" in the world, but if I make a salad with iceberg lettuce and nothing else, then there is "one kind of lettuce" in that salad.
Second, the noun that accompanies "kind" can be either singular or plural, and this depends on what you are trying to refer to. To use your own example, let's say you want to talk about how there are a few versions or styles of chocolate manufactured, such as milk chocolate, dark chocolate, baking chocolate, and so on. In this case, you would say "there are many kinds of chocolate available," because there are many kinds of a single entity (chocolate). But now let's say you buy "a box of chocolates." By this you mean that there are many individual pieces of chocolate candy in the box, and since each one is a different type of candy (peanut clusters, caramel swirls, marzipan filled, etc.), you could say there are "many kinds of chocolates" in the box.
So there is a difference between kinds of something that can be identified as a single categorical noun ("chocolate") and kinds of something that exists in numerous identifiable individual countable types ("chocolates"). Keeping this in mind, you can see that your choice of whether to use the singular or plural of the noun depends on the concept you're trying to express, as in this example: "Many kinds of car" would be referring to how there are lots of versions of "a commonly used transportation vehicle," versus "many kinds of cars" to indicate there are lots of specific models on the market.
In common usage, we often use a particular singular or a particular plural, just because of the way we usually think about and talk about the noun in question. In the case of cars, we usually are talking about the various kinds of makes and models available, so we usually pluralize it, whereas we seldom talk about the philosophical concept of a car, so we don't as frequently say "many kinds of car." But we could say either one.
The third major question in your post pertains to the use of "some." You have some confusion about how this word influences your choice of "kind" versus "kinds." But your confusion is stemming from a misinterpretation of the word "some." In this case, "some" is not being used as an indicator of a group (as in "some marbles"). This is instead a case in which "some" is being used to mean that you don't know "which particular kind." In other words, you are saying, "This is a kind of chocolate, but I don't know which kind." So, the word "some" stands in for a fairly long phrase embodying this whole concept, thus: "This is some kind (it is a kind, but I don't know which kind) of chocolate."
This happens to be a fairly specific use of the word "some," which you find often, but not always, with synonyms of "kind," thus: some kind, some sort, some type, some version. In all these cases, "some" means "one of the items in this group, but I don't know which one." "Some sort of car" means "it's a car, but I don't know which." This version of "some" can be used with other nouns when optional types of that noun are implied, thus: "She sent him to the store on some pretense (a pretense, but it is not known which one), but she really just wanted to get him out of the way."
Now, you can always use the other meaning of "some" with any group, so you COULD use it with "kinds." "In my garage there are SOME KINDS of cars." (A few different kinds.) Don't confuse the two different uses of "some."
Lastly, in common speech, people often say, "Those kind of cars...." This is grammatically incorrect, because "those" is not referring to "cars." It is referring to the kind, and so it should say "that kind," because "kind" is singular, even if "cars" is plural. This usage is very common, however, and hard to resist, but I think it's worth knowing about.