Build cask formula from source Is it possible to build cask formula from source instead of installing the corresponding pre-built app-file?
No.  

Homebrew-cask is for binary distribution.
Downloading binary and place appropriately is all what Homebrew-cask does.  

If you look in a cask (package installation script), you'll notice there's no url for source file (.tar.gz) but only for binary file (.dmg, .zip, etc.).