My little niece wants me to make her a cake like this for her birthday.  It will be chocolate cake, filled choc SMBC and crumb coated with american style vanilla buttercream.  Do you think I should I cover the cake in fondant before I adhere all those circles or should I just stick them to the buttercream?

What if...  I am just thinking out loud here...

 

Fondant was my first thought, but only if it was thinnish & well adhered to the cake to prevent the weight of all those circles from bringing it down, making it stretch & sag?  And good & dry.  Then using a good adhesive.  Maybe drying one row at a time?  Just might work, too. 

 

A thick fondant would fair better, be stiffer, but then no one would want to eat it, & it is a child's cake.

 

Buttercream might not fair much better.  How were you going to 'adhere' them to the buttercream crumbcoat?   Royal icing?  More buttercream might just slip & slide around.

 

Have you considered a poured ganache of some sort as the base coat to adhere them to?

 

You can tell I'm an eager newbie, full of hot air... also not good for the icing on a cake. lol

I think I would cover with fondant and adhere to the fondant with clear vanilla or a bit of melted white chocolate.  

AHow about some water? That is what I would use but I only have 6 years experience working with fondant.

AYou can stick them to buttercream. Start at the bottom, do a row and work your way up. There's no need to use fondant to cover the cake unless you want to.

A

Yep, nothing complicated involved at all there. Personally I would cover the cake with a thin fondant layer, but it's certainly not necessary.