You can use this method with ganache OR buttercream. I use it for ganache, always, whether I’m planning on covering my ganached cake with fondant or leaving it NEKID. And I use it with an all butter based buttercream recipe, when requested, too!
Oh, and you can use it for ANY size cake tier! (Well… any size that you can physically flip over, that is.)
This is the easiest way I’ve found to prepare ganache when using it to fill and/or ice a cake, before covering a cake in fondant, or even just leaving it as the finish! See Section 3 (Making Ganache) of the Fundamentals Course for a detailed video showing how to make this ganache.