I'm back! No getting pissed for me as I was driving but I was out which was exciting enough in itself.
Trex frosting is super sweet and not as gross as you'd think - you're just swapping vegetable fats for animal ones after all. It lacks the creamy edge that butter gives it though so it is just sweet if you see what I mean so needs a little vanilla frosting to take the edge off it for adults. Kids don't seem to mind and I have a friend who's children actually prefer the Trex frosting after trying it when she made a lactose-free cake and mine love it too.
When I did DS's ski slope (pics on profile) I wimped out and added some butter and it started off white but did seem to yellow over time so the cake that was pure white the night before the party was slightly too yellow for my liking by party morning. Next time I would definitely go pure Trex.
I think if you followed stealthsquiggle's advice to chill the cake and then re-smooth with a warm palette knife, you could probably get the cake smooth enough not to need fondant - but then I avoid fondant where ever possible as I don't really like working with it.
Sorry, OP, I have just read the instructions on the link and it does indeed sound less insane easier than the approach I took. See above comments about white buttercream - either beating like hell (will give you something white enough to pass for white as long as you don't put it against something really white, or Blathers' Trex version, which is definitely white but the idea makes me shudder a little.
Go for it. If we have scared you off completely, try googling death star cake - there are some good tutorials around for that, and it's less cake (your link is 6 layers of cake plus dome, although they are only 6" wide. My R2 had the mother of it's recipient feeding cake to random passing strangers for a week to get rid of it all!)
See expert help has arrived. Blathers is out getting pissed but says if you must use buttercream replace butter with trex and it'll be pure white but you'll need a flavouring like vanilla extract as it'll be very sweet.
OMG that is not an easy cake. It's not covered in buttercream though, that's fondant. Buttercream would just be the base layer. If you want buttercream white, add a teeny bit of milk and beat like hell (I put a timer on and leave my KitchenAid to it for 5 minutes after it has all come together).
Then get the buttercream as smooth as you can, and chill really well, then smooth again with a knife dipped in hot water, before covering with fondant
... And Good Luck. It nearly killed me, and then Maureenmlove when she did it a year later (we were both working without instructions though, so maybe we just did it wrong...)
I will link to my effort later when not on my phone
I think I can make the shape ok, but you're then supposed to cover with white buttercream, and normal yellowy colour would look wrong. I thought about buying a tub of the pure white stuff in the shops but find buying 'frosting' feels like cheating!
What would you do? Is it possible to make white icing or should I cover with a layer of white fondant? Or just buy the tub: I am pregnant and have a toddler so perhaps should be making it easy!