True chocolate ganache is chocolate and cream, or chocolate and milk+butter. If you want it to be a ganache - no lightening it up I'm afraid. You could use milk - but it will not be the same. Of course, there are other chocolate cake icing options that are not as rich.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain