At Three Tarts in Chelsea, everything has a touch of whimsy, hot steamers ($4) included. Instead of the usual shot of vanilla or other flavored syrup, the steamers here come topped with your choice of two housemade marshmallows. I tried a variety of marshmallow flavors (which change seasonally), including strawberry-basil, yuzu, and raspberry. But my favorite for the steamer is the sweet mango, which melts right into the steamer for a creamy, fruity finish. The ideal pairing? A tiny two-bite sweet ($1.70) featuring glazed kumquats nestled in a tart with frangipane filling.