Khao Niaow Ma Muang or simply Mango Sticky Rice is a traditional summer dessert because of the seasonal fruit, mango. They have the sweetest mango IMO and I managed to bring back some to Singapore! Mango Tango at SiamSq Soi 5 specialize in mango desserts but I think they are overrated. Popular a touristy area that makes it expensive as well.