How thick do you make the buttercream under the fondant, and how thick should the fondant be?

I don't use buttercream under fondant anymore since I started using chocolate ganache but when I was using buttercream, I used a mousseline (similar to a swiss meringue buttercream). I used about a 1/4 inch thickness and then chilled the cake for about 30 min to firm it up before putting the fondant on. I use a swiss fondant from Albert Uster so the fondant can be rolled fairly thin . I never had any problem with the mousseline weakening the fondant or any bulging.

Thanks for the info. Isn't ganache too soft or too think? How do you make it?

Since here people hate buttercream I try to put it as least as possible under the fondant, just to cover the cake so that it doesn't destroy the fondant... and that the cake is not seen through fondant since I tend to roll it fairly thin...

but I'd love to know more about putting ganache under too...?

I love using ganache under fondant! It can be smoothed to perfection! It firms up and stays firm, but not hard and provides a great base for fondant. Just apply like you would any other kind of frosting, smooth as best as you can. Pop in the fridge for a few minutes, then re-smooth with a warm knife. Pop back in fridge. Then smooth with a pastry brush with warm/hot water. The result can be as smooth as fondant!! Spritz lightly with water and apply fondant the same way you would any other.

It seems like a lot of effort, but really goes very quickly, and the results are TOTALLY worth it!!

The ganache I use is fairly simple. It's a ratio of 2 parts dark or milk chocolate to one part cream or 3 parts white chocolate to one part cream. There are lots of threads on CC that discuss how it's made and helpful techniques. You want a fairly soft consistency ... a bit thicker than swiss meringue buttercream, I should think. It makes a fabulous undercoat that supports the fondant but best of all the taste of the chocolate against the fondant is divine. The cost of the chocolate makes the cake a bit more expensive than using buttercream but for special cakes such as weddings and very special celebration cakes, it's worth it.

Thank you I'll definitely try it...