I've seen similar cakes on this website. Maybe you could search for bark and send a PM.
There was a previous thread (maybe a month ago?) with this same cake picture. I don't remember what the result was, but maybe they found out and you can find the thread. I remember the cake because I said I thought it looked like wounds... so maybe you can search my name and posts that contain "wound" and find it that way!
If I had to try and replicate that cake I would cover it with brown buttercream, refrigerate, then cover very, very lightly with off-white buttercream. You could build up the brown buttercream for the "knots" and smooth some white over the "knots" lightly with your finger.
Then, after refrigerating again, I'd use a rough toothpick to "gouge" out the bark lines. You could make the "puffy" look by digging the toothpick slightly sideways under the gouged line to make it "puff out". (Similar to what you do when making nostrils on a gumpaste face.)
Does that make sense the way I tried to explain it?
Apti, i totally makes sense what you are saying. My initial thought is that it is butter cream. I think it is the colors that are throwing me off. I am wondering if there is a base coat of black underneath so that it looks like the scaring on a birch tree. I will search the forums and see what i can find.