How to recompile only the iOS App Extension?

Question

I am working on a iOS keyboard extension, build times of the main target are very slow and consequently the build time of the Keyboard Extension is very slow. I've tried removing the main app as a build target from the scheme in the hope of not recompiling the main app and just rebuilding the keyboard extension but this doesn't seem to work instead an old cached version of the keyboard extension is still present?

Any ideas how I can do this?