The chocolate is really interesting hint in this drama. This post will contain my thoughts, theories, possible meaning of chocolate in the drama and culture. I will try my best to consequently update this post with more and more tropes, because now, the last released episode was 8 (4 in 1 hour format which I will use here) Also, I need to warn you: I love over-interpretation and looking for second bottom, but this subject seems legit to me and I think that for many of you too. Let's start.

1. Meaning of chocolate in dictionary: 

chocolate (noun)
choc·o·late | \ ˈchä-k(ə-)lət  

Definition of chocolate:
1: a beverage made by mixing chocolate with water or milk

2: a food prepared from ground roasted cacao beans

3: a small candy with a center (such as a fondant) and a chocolate coating

4: a brownish gray.

Also, sweet, joyful thing, an aphrodisiac.

2. Metaphor; meaning: 

My personal thoughts and people's opinion found in web:

a) guilty pleasure, something addictive; desire of more; guilt; joyful; complexity; form of a trap; something you don't want to share with others,

b) want to be loved; love,

c) celestial thing, a gift from God,

d) something, that makes you feel better when you are upset, the sweetness,

If you found another meaning, please share it too! 

I subjectively highlighted the most possible things in the context of drama.

3.  Possible meaning in "Angel's Last Mission: Love":

I will start here with my theories, some of them could be not related with chocolate but after all, I think it will be summed.

1. Ji Kang Woo knows about Dan's real identity:

I know that it might sound crazy (I don't believe it in 100 percents) but there is one suspicious thing:

a) in episode 2 (4 in different order) anyone from outside can simply saw Dan's wings through window. Ji Kang Woo came into the Ivy Mansion in really short time after they disappeared. It was pretty obvious for me when I rewatched this scene, hope I'm wrong.

2.  JKW's scenes with chocolate and how he 'infected' Dan with addiction to the chocolate: ( this title sounds a little bit hilarious for me, I'm truly sorry :D )

a) the beginning of realization the plan concerning Yeon Seo.

After receiving a call about Yeon Seo, he decided to approach to his 'project'.

b) Our 'sweet' story started in a bakery (if I didn't ignored something) when JKW decided to buy a chocolate cake in order to visit Ivy Mansion for the first time.

Sadly, he wasn't able to give it back then, but...

c) In 3th episode, Kang Woo gave Dan the same cake.

His words were really interesting and caught my attention. When all of them get out of the car, Kang Woo said, that he 'wants to get know' Dan, calling the box a bribe, adding that he always can purchase more. The part with 'bribe' might seems like a joke, but it low-key worked with Dan. Why? Let's go to the next subsection.

d) Dan and his (probably) first time with chocolate.

The addiction started here. I also found a second bottom in this frame.

e) Next scene, Kang Woo is watching a live transmission with Yeon Seo and Dan with... Pralines.

f) Scene in the café with Dan and Ni Na.

g) Yeon Seo and Yoo Mi in Dan's room.

Let's clean up the mess I've created here and put everything clearly. 

I have some thoughts and here they are:

If Ji Kang Woo knows about Dan's identity, then his operation (with that cake) could be planned and a chocolate must has a special meaning for their being/identify as an angel (not only as a metaphor, but in other special way, I'm going to stay focused about the first thing). Being addicted to something (for example to chocolate, or just to other person) don't seem like a heavenly thing. It's associated with limitations of human being. The cocoa sweets are reference to become more and more like a real mortal, like the feelings that were included as a chocolate's metaphors. Desires, worries, sins, habits, happiness, romantic love, tort and further - it's the essence of our lives. Angels (not fallen, which Kang Woo may be) are the perfect, innocent, good and fair creatures in our minds. Chocolate could be a symbol of the blurred boundary between being a human, and an angel; discovering emotions in Dan's case, but in Kang Woo's - complete abandonment of his old, angelic values and legacy. We can also ask a question: cake was a metaphor of trap for Dan? Does it mean that Kang Woo will pay attention to him? I think we can't fully reply to that right know, because JKW bought a chocolate cake (I'm certain just because he liked it) before he might saw him in Ivy Mansion, but his words while giving Dan second pie were suspicious.
The frame with dog and cake - I'm sure it's just me overreacting but I will still stick to my opinion about second bottom in this scene. The dog was ill after he ate this pie, which is normal thing, but the whole meaning of that might be 'someday you will get hurt because of it', and here we go again with my whole trap theory.

I'm almost sure that writer will surprise us with the real meaning of this whole cocoa-plot, but there's one thing left: in my subjective opinion, JKW's obsession with pralines may be also an expression to his mania with Yeon Seo. He seems to be quite obsessed and possessive over her. Also, we learned in chapter 4 (8) that he had a though past and he is/was an angel.

I found someone's post about mention that Yeon Seo likes chocolate. I think that I completely ignored this fact while watching, but: I think if he really chose the chocolate cakes because Yeon Seo mentioned in the video that it makes her happy, after all it's not the main reason and have more depth, he is too much obsessed with chocolate in my opinion. Also, he wouldn't give the second cake to Dan.

I'm looking forward for your theories and thoughts, I'm also curious is there anything you disagree with me. :)

wow your ideas are so interesting. I also think That Ji Kang Woo knows about the true identity of Kim Dan. That's why he seems to want him being far from Yeon Seo.

Wowwww! I love the detail you've put into this. Can't wait to see how it develops as the episodes go. Will be keeping up with this thread!