To enable Font Awesome for iconCls....

[chrwei]

If you want to use Font Awesome for iconCls icons, add this to your index.css then use the normal "fas fa-xxx" classes in iconCls.

Code:

.l-btn-icon {
  font-size: inherit;
}

I've tested minimally, this doesn't seem to affect the regular icons, so you can use either.  there's probably some case that does have an effect, so test your stuff