Learn Finnish: Pojan tekee mieli suklaata.

In the tekee mieli (craves / feels like) structure, the person who has the desire is called the experiencer. In Finnish, this experiencer must be in the genitive case. So, poika (the boy) becomes pojan.

If there were multiple boys craving chocolate, would the verb tekee become plural?

No, the verb stays exactly as tekee. In this structure, the grammatical subject of the sentence is actually the word mieli (mind/desire), not the boys. Since mieli is singular, the verb is always the third-person singular tekee, regardless of whether the experiencer is minun, pojan, or poikien (the boys').

Why is the object of the craving, suklaata, in the partitive case?

With the tekee mieli structure, the thing you are craving is always in the partitive case if it is a noun. This makes sense because chocolate is an uncountable mass noun here, meaning the boy desires some amount of chocolate rather than a whole, finite unit.

How would I change this sentence if the boy craves to eat chocolate?

You would add the first infinitive (the dictionary form) of the action verb into the structure, making it: Pojan tekee mieli syödä suklaata. The noun suklaata stays in the partitive because it is still an uncountable mass noun acting as the object of the verb syödä.