How thick of a layer of buttercream do you put on under fondant?

A very thin layer over the cake to crumb coat it. You want a smooth surface to lay the fondant on.

I use more than a crumb coat, but less than if it was iced only in bc. You may want to search the archives, there has been a lot of discussion about this. I think it really boils down to trial and error to find out what works for you - everyone does it a little differently. Hope that helps!

Bec

I do a basic crumbcoat, then when that dries, I'll do another thin layer of buttercream... same as i_heart_pastry...

HTH

I find that doing a nice layer of butter cream and smoothing out just like when your icing a regular bc cake and putting it in the freezer just until the butter cream is pretty firm really cuts down on the "cellulite" that soft butter cream makes. Also most people peel the fondant off, so I'd like there to be frosting under so it's just not cake and fondant.

i do a slightly thicker than usual crumb coat.. not super thin, but not as thick as if it were just bc...

same as I---heart---pastry

Same as Wendy LaLa

I trim my cake sides to make them straight; apply CC of BC; zap in freezer a few minutes and then cover with 3/16-1/4 inch fondnat.

HTH

Nowadays I do the crumb base, and in the freezer, then i put a thicker layer to get a smooth surfuce, in the freezer again and I also smooth it out with a roller, then i get a really neat surface and when the BC is frozen and you are lucky enough you get a second chance if the Fondant does not fit properly without wasting the fondant.
However i do use a thinner layer of BC than i would with BC only.