This is a sauce, so it is not thick enough to use as an icing on a cake. If you need to decorate the cake, you could put some cocoa or icing sugar in a fine sieve and sprinkle it over the top for something. I would then cut the cake and drizzle the sauce over each piece...or pool it on a plate and put the piece beside/on top of it.
If you want a thicker icing to spread over the cake, you could use the ganache from Step 1 in this
Chocolate Ganache Cake recipe. Hope this helps!