Boil the mangoes in a large pot of water for 15-20 minutes until their skin has burst and they have softened.
Remove the mangoes from the hot water and cool to room temperature.
Peel the cooled fruit and extract all the flesh in a bowl. Squeeze the stone to yield all the squishy pulp around it.
Add sugar and salt to the pulp and whizz with an immersion blender (you can also do this in a food processor), till the sugar has dissolved and you have a thick, soupy liquid.
In a jug, dilute this concentrate with 700 ml cold water, season with cumin powder, mix and chill till ready to serve.