Using a knife, make several small slits in the mangoes.
Add 2 cups water into the steel inner pot, then place a steamer basket or the wire rack that came with the pressure cooker into the pot. Place the green mangoes on top of the rack.
Secure the lid, close the pressure valve and cook for 15 minutes at high pressure.
Open the valve to quick release any remaining pressure.
Allow the mangoes to cool down, then remove the peel.
Place the pulp into a blender along with the 4 cups of water, sugar, black salt, roasted cumin powder and salt and blend until smooth. Taste the drink and add more spices according to taste.
Pour this drink into glasses filled with ice. Sprinkle a little roasted cumin powder on top if desired and garnish with a fresh sprig of mint.