Hi, again, @knwalter - This phenomenon is most often referred to as "cratering", and we have several forum topics on the subject already. Here's one of the most relevant ones: https://cookieconnection.julia.../topic/icing-craters
If you do an "advanced search" of this site (click on the magnifying glass icon and then "Advanced Search" above the box that opens up) for "crater", a bunch of other forum topics on this subject also come up.
Hope this helps.