What about очень вкусное?
To me, "не такое вкусное" implies comparison (with some other type of cookies or another batch I had some time ago).
"Не очень вкусное" is an absolute statement - it's just not very tasty, full stop. No comparison is implied.
Well, technically очень is "very", and that has slightly different meaning. But I think in this case it would still be correct.