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Here’s some information in a hundred-year-old cookbook about making cocoa and chocolate. Not quite sure how cocoa differs from chocolate.
Cocoa and Chocolate
Theobromine is the stimulating element in cocoa beans, and is much less pronounced in its effect than the corresponding principles in tea and coffee. The high percentage of fat, together with other food principles, places this bevarage in the class with foods. As a rule, when making cocoa or chocolate, follow the recipes found on the package. It will be well to bear in mind, however, that boiling will greatly improve it. Beating constantly with an egg beater while cooking will thoroughly mix the ingredients and prevent a thin skin from rising to the surface.
Mrs. DeGraf’s Cook Book (1922)
A Hundred Years Ago 
I think cocoa and chocolate are the same thing. I’ve heard that favorite winter beverage called cocoa or hot chocolate. We should have been more inventive with names. Since the drink conjures up a snowy ski lodge, it could be Ski Choc.
Before instant hot cocoa was either available or was too expensive, my mother would stir together the cocoa powder, milk (and it was always whole milk), and sugar and then stand there stirring with a whisk until it got very close to boiling. Immediately she’d portion into mugs. It was better than any of the gourmet mixes available… I remember my first date – she had the hot cocoa nearly ready when I arrived home and she invited my date in for a mug! (as for the date I don’t remember much else)
cocoa, powder, chocolate, solid, like baking chocolate.
I wonder what the difference between the two would be too.
In making cakes, pies and the like, I prefer cocoa powder over the solid chocolates… except for chocolate chip cookies, then, well, hey, chocolate chip cookies wouldn’t be the same without the chips. š