I am familiar with Serendipity, and it's very good.
But yesterday I went to Jacques Torres' shop on King, (I've been before) just to try his version of this treat. $4 buys you a VERY good sized cup of gorgeous frozen hot chocolate, in either the plain, wicked (spicy), banana, mint, orange, or peanut butter variations. I had the orange - real small bits of candied orange inside the frozen hot chocolate, with a big gob of real (not out of a can) heavy sweetened whipped cream on top.
Unbelievable. It's really just unbelievable. I'm going back today to try the mint variation. It may be one of the best things I've ever put in my mouth.