Aare you sure you posted the right image? ;)
YES, SO SORRY Totally posted the wrong picture. Now how to delete the guys photo.....
Abeautiful cake, I love the rustic-ness of the Hessian too. my suggestion would be using the back of a dessert or soup spoon to make the waves rather than a spatula or palette knife. I don't work with buttercream much, but i would chill it for a small amount of time once you had the cake covered and evened out (i think smoothing it totally would be pointless) I would let it harden just a little so you have a bit more resistance to get better ridges
I think it is a pretty cake too. I agree that a soup spoon would probably work.
I'm sure there are easier ways, but this is how I do it. Crumb coat, then a good thick coat of butter cream. Then I use a small off set spatula, and using a rocking back and forth sort of motion with my wrist, I create the 'swoops' in it. Now and then adding some more frosting just to add to the texture.
I don't refrigerate, except for the crumb coat, because if the icing is too hard you won't and up with a nice soft looking texture. It gets a bit rough, and you have to use more pressure, which can dislodge some of the crumb.
It's called "intentionally messy" and I took Josh Russel's free buttercream class on craftsy and he touched briefly on it. It was a good class. Lots of good tips. Not anything significant but good for free. Basically scrumdiddlycakes has a good description on how to do it. But, I'm sure if you youtubed "intentionally messy buttercream" you'll get a few video hits.