What can I do with solid cocoa?
I'm no expert in the matter, but the only use I was really able to find is to make Koko Samoa.

This blog I found to be very informative on the subject: https://chocolateclass.wordpress.com/2015/05/13/koko-samoa-simple-recipe-complex-history/

Basically, you shave the cocoa in small pieces, and infuse in hot water, add some sweetness, and you get a nice drink that you serve hot!

Here's a video explaining: https://www.youtube.com/watch?v=z3wjufJUBhI

And according to this company, that sells that product, you can make large batches and reheat the next day: http://www.kingkoko.com/recipes.html 

Interestingly, their website have a "recipes" section containing only a single recipe for koko samoa. Which encourages me to say that product doesn't have a lot other practical uses.