La Tortuga, it can't possibly be that the 6th cookie tasted as good as the 1st. Your marginal utility per cookie consumed will always fall with each successive cookie.
Take Snickers bars as an example. I really enjoy that first Snickers bar in a day. It gives me, let's say, 14 utils of consumption delight. The next one I eat that day is also really good, but I get 10 utils of delight from it. The third is merely "good", and I get 3 utils of delight from it. I sorta like the 4th Snickers I eat that day, gaining 1 more util of delight. The 5th -- I am indifferent, gaining no delight at all.
You'd have to pay me to eat the 6th Snickers bar that day, meaning that the marginal utility for me has gone negative.
Of course, the PBJ family cookies were probably still providing very positive marginal utility for you, even at the 6th one. But if you increased that, your added utils would surely go negative at some point (maybe a few batches).
Hmmm. Is this an experiment we need to test?