What is a Cookie Stack Match-3 Elements? These are boxes covered with a ribbon, and you need to match pieces beside it three times in order to open them.
Cookie Stack (Source: Playrix.com)
Once you have done so, you'll get three cookies. This is an added difficulty if one of the requirements of the puzzle is to collect cookies, so try opening a lot of stacks that you can within your move limit. Using power ups to activate multiple stacks will help.