Meaning and etymology of the expression “棒棒的！” Someone wrote to me "两个月就写这么好，棒棒的！"

I get the first part (though I don't think it's true - 哪里哪里 ；-））, but what exactly does that "appendix" stand for? I interpreted it as a kind of "clapping one's hands in approval", as Google Translate just spits out "Bang Bang!", but that may well be too far-fetched!

Any input on this?
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.