Place whole mangoes in water in a saucepan and bring to a boil. Simmer for 10 minutes then remove from heat. Press the mangoes in the water until they are soft. Remove mangoes from the pan (do not discard water), cool, and peel. Remove the mango pulp with a spoon and blend it with the water until smooth. Place mango pulp in a saucepan with the sugar and cook until the sugar is completely dissolved, stirring constantly to keep the mixture from burning. Remove from heat and add the cumin and salt. Cool completely. For each serving, place 1-2 tablespoons of the mango mixture in a glass and fill 3/4 full with ice water. Finish with finely chopped mint leaves and crushed ice and serve immediately.