I was a huge devotee of the J'adore cafe on 7th Avenue before it closed - I'm assuming the 23rd street location carries most of the same products. The desserts are dangerous, especially the sugar cookies, if you get one don't taste it until you're too far away from the store to go back for another :). Cookies, pastries, cupcakes, sandwiches, breakfast, coffee, everything I've tried has been great! (and the red velvet cake that appears every so often must be good too, as it's always gone before I get to try it!)
There's also a sign saying to ask about their kosher and sugar free deserts. I haven't, but given their high standards for the rest of their menu, it's probably a good bet.