chocolate cake made with cocoa powder (not sweet), but rich in flavour.
you buy the chocolate powder (sweentened), most of usage for beverage.
of course, you can use it to make the cake, but the flavour not feel good. Because the drinking chocolate ususally for drink, u can try, ideal use more in quantity and reduce some cake flour. e.g. 100g cake flour for cake, 80g cake flour and 20g chocolate powder for chocolate cake.