Food and diet is a complicated thing that a lot of people love to oversimplify.

It's important to eat your fill, your body needs food, and it's not an intelligent idea to deny your body the energy it needs to function. But the other side of that coin is that it's also important to eat the right foods, you need to avoid super processed hyper palatable sugary foods that are everywhere in todays world (like the cookie you mention).

The problem is 100% what you're eating, not how much you are. If you eat the right foods (which of course is a huge complicated thing in and of itself, that you'll need to research), you can safely listen to your body and trust what it tells you.