Before putting on any icing, lightly brush any loose crumbs from the cake. Then take a small amount of your regular icing and thin it out a bit with a little extra water. Thin icing spreads easily. Thicker icing will drag across the surface of your cake and create more crumbs than necessary.
Next, let the "dirty" layer set. Depending on the humidity in your home, this may take an hour or two. The icing should "crust" a little so that the crumbs remain caught in it. Once it's set, you can smooth a second layer of icing on the cake.
See? Much smoother and you don't see all those chocolate cake crumbs. :)
If you want to see how this cake looked in the end, check out this post.