See below...

“Trying to make the best chocolate chip cookie and wondering why in your YouTube video you melted both and Brown both sticks of butter versus your recipe in the book only does one I want to kill the butter down so that I don’t get a greasy cookie so just wondering why you did both versions” - @firehouse

Funny coincidence, I answered this question in the guide already, see the rec titled "On browning butter in chocolate chip cookies...".

To summarize, you can best avoid greasy cookies by only browning half the butter, it's just that sometimes I forget and end up browning all of it.