Create ascx page based on this:
http://devnet.kentico.com/docs/devguide/index.html?developing_web_parts.htmAdd your button, setup visible to false, and add this in code behind:
if (CMS.CMSHelper.CMSContext.CurrentUser.IsAuthenticated())
{
you button.Visible = true;
}
There is another quick way.... :)
Add Static HTML WebPart... inside add you link and in WebPart properties setup Display to roles, to display it to roles you want...
Hope this gives you idea...