Step 1

Toast cumin in a small dry skillet over medium heat, shaking pan occasionally, until slightly darkened and very fragrant, about 2 minutes. Let cool, then finely grind in spice mill or with mortar and pestle.

Step 2

Whisk lime juice, kala namak, and ground cumin in a small bowl until salt is dissolved. Pour into a large pitcher and add yogurt, mango pulp, and 2 cups cold water; stir until smooth. Chill until cold, at least 1 hour and up to 1 day.

Step 3

To serve, divide lassi among glasses filled with ice and garnish each with a mint sprig.