Meaning and etymology of the expression “棒棒的！”
棒 means 'great; awesome'.

棒棒 the repetition basically makes it sound rhythmical and cute.

的 after a repeated adjective in spoken language is a very subtle thing - it slightly changes the meaning from 'awesome' (praising your writing) to 'keep being awesome' (encouraging you). 

Overall, the difference between (真)棒 and 棒棒的 is like that between '(very) cool' and 'cool beans' in English.