Ты такой классный (to a guy)
ты такая классная (to a girl)
The words классный/классная and хорошенький/хорошенькая are very similar in meaning, but there is a difference.
хорошенький/хорошенькая implies that he or she is good-looking or even attractive.
классный/классная more implies that he or she is in your opinion an interesting person, you enjoy to talk to him or to her very much, although it also may refer to appearance.
ты = you
такой/такая = so
cute = классный/классная or хорошенький/хорошенькая
we generally don't use the verb 'to be' in the Present tense, so there's no word for 'are'
Let me also give use a piece of advice. Althought it's nice to know how to say it, never use those phrases, unless you know the person you're talking to rather well.