This isn't a recipe for piping flowers.  To pipe chocolate flowers, use the ganache straight.  Chill it and then blend up in a food processor to get a good piping consistency.

This recipe copy has lost the weights of the ingredients: cocoa can pack down and mess up recipes.  So try sifting it before measuring.

Otherwise, you could try premixing your cocoa with the liquids in the recipe and letting that sit for an hour.  That will help even out the consistency.  I will have to check when I get home, but I guess the original recipe might not require the ganache to be cold when you add it to the icing.

Toba's recipes are based on standard bakery technique of spreading and immediately smoothing with a straight edge like the 11" Wilton straight spatula.  You can use the DRY hot knife as long as you do all work immediately after you make the icing.  

Then let the rest of the icing set up a bit at room temperature  for the borders.

And should you really be interested, Toba's books are all great value with amazon discounts.

You could try the upside-down frosting method, if you need a perfectly smooth top.

Here's a cc tutorial: http://cakecentral.com/a/upside-down-icing-technique-for-perfectly-smooth-icing

I do have a version with the weights (it's what I use...hate sifting...lol).  I am going to try the upside-down method this time (I did a half-sheet cake with it the last time, that is just way to big to flip around for me ;) ).  I may need to make some "standard" types of chocolate buttercream to get through the flowers in-class, though.  I'll figure out something!  Thanks for the tips...I will keep trying them!

Well..I used this again, I was able to get it smooth :D  I think making a round cake and working fast made it easier.  It also seemed a bit softer this time (I know we used a tad less sugar, and the butter was room temp as was the ganache).  Unfortunately, the room-temp buttercream was too soft to pipe really well (it was fine for the dots, not so much the rosettes).  I should have put the piping bag into the fridge before we left...

Just keep trying, trying, trying :D

I know this is late for the original issue, but...

Toba Garrett has revised the instructions in her newest book "Master Class".

The buttercream recipe calls for the same amount of ganache at room temperature "LEFT TO HARDEN" This should help the consistency stay stable after it has been mixed in.

I believe that chilled ganache brought to room temperature would also serve. 

And the book is full of spectacular very sophisticated cakes (only 1 half-dud) plus the usual panel of recipes. I'm so glad the library just got a bunch of copies.  Worth the discounted at Amazon price...