Having decorated cakes with buttercream for many years Paul had his moment of enlightenment when Liz Russell came to work for him. Liz had worked with Planet Cake in Australia where, mainly because of the heat, they used Chocolate Ganache to cover their cakes.
Its more expensive to make, but once you've started covering cakes with this you won't want to go back to buttercream. Dark Chocolate, with a higher level of cocoa is the best for stability, however there's a slight bitterness so check that the intended audience are happy with that.
Applying layers of ganche as seen on the Tutorial Library will ensure a very sharp and smooth covering for your cake. It removes any bulges or bubbles that can often be caused by buttercream.
Here's the recipe Paul uses:
1 Pint of Double Cream
2 lbs of chocolate (buttons are recommended)
Method: Place the chocolate in a clean dry bowl. Bring the cream to the boil, remove from heat and pour over chocolate watching out not to splash yourself. Leave for 2 minutes then stir until blended and glossy.
Ganache will keep refrigerated for one month.
This and many more recipes and cake decorating help is available on the Tutorial Library.