What makes molten chocolate so much more filling?
Having done something very similar the other day, I think the banana  accounts for more than you think (you don't say how big your slices were, but 3 big slices could easily add up to the whole very small banana I coated in chocolate left over from dipping  these biscotti).  The butter will make a tiny bit of difference unless you used loads.  I don't add any when dipping, and chocolate is mostly fat anyway. Fibre in the banana is filling, and fat from the chocolate is satisfying, so the combination makes you really feel like you've eaten.  

Another factor is that dipping cookies uses up surprisingly little chocolate, and chocolate looks like less when liquid so you may well have had more chocolate than you thought.  By the time you dipped the banana it might have thickened a little so you could scoop a nice thick layer.

You didn't by any chance have a few mishapen/broken cookies in the batch that needed "testing", did you?