What could a cookie possibly mean? Unlike candy and marshmallows, cookies are crunchy. Apparently, this is taken to mean a "casual" or "dry" relationship. Thus, it is said to mean "you are just a friend".
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There are of course some exceptions to the above categories. Although macaroons tend to be classed along with cookies, as they also have a slightly high class feel to them, they are said to express that the recipient is someone special.
That feels very on brand for Douki-kun. Can’t give her anything like hard candy (a confession), but also can’t give her a ‘just friends’ gift and sabotage things either. Just enough plausible deniability to be safe.
Going to the office on a day he didn’t need to be there, just to make sure to give her that “You’re very slightly special to me (probably)” gift?