You can use real chocolate, not cocoa. I do this all the time.
First, what others posted is a factor. If they were standard size, 15 minutes may not have been enough. Coupled with the fact that almost all ovens are off in temp, this could be your downfall.
But there is one other possible culprit. I'm assuming that the recipe is scratch because of the chocolate. Every recipe is based on a certain % chocolate used by the author of the recipe. For example, I stock 60% chocolate all the time. One of my regular cakes uses 70%. I didn't have any and used the 60%. It sunk. I baked it again and it sunk again. Finally after researching, I found that even a slight change in % chocolate used can cause a recipe to fail by sinking. The farther off the %, the more sinking happens. The problems come when a recipe calls for chocolate and does not specify which kind. The recipe works for the author of the recipe, but without this information passed on, some recipes will sink.
Many recipes will work when changing out the chocolates, but if your recipe sinks after making it correctly, try using a higher % chocolate.