Render transparent shadows (cycles)

Hello everyone

I know this topic has been discussed many times but seriously I tried everything for hours and still dont get it!
To simplify my problem, lets have a look at the following scene:

I would like to render only the cube with its shadow on the right (on the plane, transparently). The plane itself should not be rendered. How do I get that result? I tried light path, masking different layers, ray visibility etc. It doesnt work, maybe because I use meshes as lights?

Thanks in advance!!!

shadowrendering.blend (566 KB)

Thank you very much!! It worked very well in the simple scene.
However, in the scene I try to implemet it, I dont get the same result…
Can you give me an advice why it doesnt work?
Blend-File: (solved)
I would like to have rendered only the goldbars (with environment-reflection) and its shadows.

PS: here the solution to the simple scene that worked well:

Alright, I solve the problem in my post before, I get now the result I want, except one little thing:
The plane on which the shadow falls (second layer in the blend) leaves a thin shadow which darkens the scene later in the movie with real footage. I did the same setup as in the basic scene (shadowrendering.blend) and in the basic scene the plane doesnt leave that think shadow on the ground everywhere. What did I wrong?

Blender file (for v2.71):

I would like to get rid of the shadow (red arrow) and just have the object shadows (blue dots)

I tried again to solve the problem for hours… no solution yet… somebody has an advices for me?

The easiest solution would be this: