Desserts near the Diamond District?
Teuscher chocolates are fantastic, especially the champagne ones (their specialty). Michel Cluizel also has a relatively new outpost on 5th ave. I personally think both are better than Maison du Chocolat.
A completely different option would be to try one of the food trucks; many of them are in midtown during the day, including some dessert ones such as Wafels and Dinges and Treats Truck.
620 5th Ave, New York, NY 10020
584 5th Ave, New York, NY 10017
The Modern Bar Room has great desserts.
La Maison du Chocolat in Rockefeller Center has great chocolates and macarons.
Financier Patisserie has a branch on 48th and Sixth.
Magnolia Bakery has great banana pudding, though the cupcakes are debatable.
9 West 53rd Street, New York, NY 10019
La Maison du Chocolat
30 Rockefeller Plaza, New York, NY 10020
1240 6th Ave, New York, NY 10036
1211 6th Ave, New York, NY 10036