Note I have now restored Cookie 6.7.4 from my Time Machine backup and it goes straight in with minimal CPU as it used to.

There is no ~/Library/Containers/Cookie/ but I found and deleted these:
com.sweetpproductions.CookieApp
com.sweetpproductions.CookieApp.Helper

Cookie then gave me a series of questions regarding upgrading to 6 (but I must had had 6 already, right?) and asked permissions etc. but still sits spinning (55% CPU).

I have the same issue - Cookie updated itself to 6.8 overnight and now sits unresponsive and taking massive amounts of CPU. Restarting it, and rebooting the Mac makes no difference. I tried removing the sandbox permissions which has had little effect other than Cookie now sits taking just 90% CPU instead of 200+ as before (4 core i7). This is under MacOS 10.15.7 - the latest that this 2013 MacBook will take. I am having to run with Cookie disabled now in order to shut the fan up!