Not "dissolve" the chocolate, "melt" it. You break the chocolate into chunks in a bowl, then put that bowl into a larger bowl filled with hot water, and the heat is communicated to the chocolate to melt it without any danger of burning it, so that you can then put it into a mold and/or beat stuff into it. (A double boiler, in other words.) Chocolate has a ridiculously low burning temperature, and IIRC it gets bitter when overheated, so you have to do something like that unless you think you can do an absolutely perfect job on the first try. What with how common this general topic is in manga, this is the third time I've seen a translator make this particular mistake.
(And, as with making cakes and cookies, although it can be difficult to make chocolates which look good, just following the recipe to make a basic product is nowhere near as difficult as manga makes it seem.)