'Could' is often used when there is a condition that must be fulfilled in order for the thing to happen. For example: 'I could eat the whole packet of biscuits, if I was hungry' vs 'I can eat the whole packet of biscuits.' In this example, saying 'could' implies that you can eat the whole packet, but you probably wouldn't do that because it would be extreme. Using this word 'could' emphasises how much you love these biscuits. But if you were to use the word 'can' here, it would be more like just stating a fact. Another example: (common British expression) 'I am so hungry. I could eat a horse'. (This is how hungry I am) vs 'I am so hungry. I can eat a horse'. (I am literally able to eat a horse) Hope this helps!

COULD sounds like you really like them, but aren't going to eat the whole packet. CAN sounds like you have a plan to do so, and are going to eat the whole packet. Also, FYI, in the U.S. these would be COOKIES (biscuits are something completely different in the U.S.)

COULD - hypothetically speaking CAN - ability / possibility