Ahh, Magnolia. This bakery was founded in New York in 1996 and is often credited with starting the cupcake craze. Their cupcakes are beauts, a pretty decent size for an American cupcake (aka on the smaller size) with simple buttercream icing decoration.
I've tried a few of their cupcakes but what really got me hooked was their Carrot Cake. It's a hefty slab of a thing (here they go back up to US sizing) so make sure you've got room to handle this thing. I've been in there at off peak hours and it's never quiet, be prepared to wait a bit. Their Midtown location is right at 30 Rock, pick up a treat and take it across the street for some nibbling and people watching.
Rockefeller Center, 1240 6th Ave