Any existing element (which probably won’t be useful, as an icon element won’t act like a button) or any custom element you write.
Yes, this is broken since HA 0.70. I haven’t figured how to re-implement it yet.
You should either use
extra_html_url: ...
extra_html_url_es5: ...
or custom_ui: local. Not both.
Your files are at the correct places, but it is not clear to what is not working.