To do this in fondant, I would suggest covering the cake and then painting by hand. Different sized brushes (with different amounts of dye) could give a more random and realistic look for the wood too.
The picture does look like a ganached cake smeared with buttercream. I agree with cakeartist86, if she wants it in fondant then cover in fondant and hand paint it. I guess you would need to cover it in white And then paint on the brown. I'd be interested to see how that turns out. Good luck
I just saw this cake on Jennifer of Cup a Dee Cakes' blog and she used fondant. I know it's not exactly the same as what you have here, but thought it might help to see another cake like it done in fondant.
Instead of straight dye, I'm thinking about painting the fondant with many different shades of brown, thinned buttercream icing, so that it gets that '
"textured look" she also wants me to add some burlap fondant flowers to it. She is really stumping me today... Burlap fondant? I would use a real piece of burlap to transfer the texture onto the fondant, but I am afraid it would not be edible then? How else would I get that "burlap" look?
I have seen an ' impression mat ' that would give the exact look of burlap in hobby lobby - in the clay section! Or you could just use the edge of a metal ruler or something similar and just make lots of lines in the pattern?