have gameobject allways rotate to point

Hello, i try to make my menu always facing the camera.

however only the z axis is supposed to change so it only points on the z axis to the camera. While x and y rotation of the gameobject always stay 0

here is what i tried, but it is not quite right:

transform.LookAt (Camera.main.transform.position, -Vector3.up);
        transform.localRotation = Quaternion.Euler (0, 0, transform.rotation.eulerAngles.z);