For the Palm Sugar Syrup: In small saucepan, combine palm sugar with ½ cup (115ml) water. Over medium heat, simmer palm sugar and water mixture until sugar has just dissolved. Skim off any scum that may have floated to the surface of the syrup. Reserve ½ cup (115ml) syrup for the lassi; refrigerate remainder for future use. The syrup will keep indefinitely.
For the Lassi: In large mixing bowl, whisk together yogurt, buttermilk, mango purée, palm sugar syrup, kosher salt, and lime juice. Serve immediately over ice. The lassi will keep in the refrigerator for 7 days.