Dessert around Sherbourne / College (or possibly further)?
There are many restos, coffee shops, and pubs pressed together in this tiny strip. I'd suggest you just grab your partner, wander and read menus, then go wherever strikes your fancy.
Personally, I like Byzantium. George Bigliardi's used to have great desserts, but I've not been in a while. I'm also quite fond of Hair of the Dog (great atmosphere for a date BTW). Food's good, drinks are great, I've not had dessert there. Been reading good things about Fuzion, but have not been yet.