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1
Place the whole raw mangoes directly over a medium gas flame and roast, turning with tongs every few minutes, for 20–25 minutes, so the charring skin acts as a sealed jacket that steams the interior to complete softness rather than simply scorching the outside. This is where the dish lives or dies: the exterior must char thoroughly and the interior must steam until completely soft — press the mango through its skin; it should yield without resistance, and the flesh inside will be pale, faintly smoky, and tender. Do not wash the mangoes after roasting, as the char clinging to the skin carries the smokiness you are about to peel into the flesh.
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2
Allow the roasted mangoes to cool until handleable, then peel away the charred skin with your fingers and discard it. Scrape all flesh from the stone and collect it in a bowl, working deliberately so none of the smoky, yielding flesh is left behind on the stone. Do not rinse the flesh — water washes away the faint char note that this open-flame method is specifically built to deliver.
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3
Dry-roast the cumin seeds in a small heavy pan over medium heat, stirring constantly, until they darken by two shades and release a sharp, toasted fragrance — approximately 2 minutes — so the raw grassy edge of the spice gives way to the deep, almost bitter warmth that will anchor the drink's finish. Remove immediately from the pan, because residual heat in the pan will continue to cook the seeds and push them past the point where they smell deep and smoky into the point where they smell burnt. Grind to a fine powder.
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4
Combine the roasted mango flesh, sugar, black salt, ground roasted cumin, and fresh mint leaves in a blender and blend to a smooth, uniform concentrate, so all the aromatics are fully incorporated and the mint is bruised thoroughly into the base rather than floating as a garnish. Taste the concentrate: it should be intensely sour, salted with a pronounced mineral note from the black salt, and aromatic with mint and cumin — it is not intended to be palatable neat, and its aggression at this stage is the measure of how well it will hold its character once diluted.
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5
To serve, dilute the concentrate with cold water at a ratio of 1 part concentrate to 4 parts water and stir well, so the concentrate distributes evenly rather than settling in a dense layer at the bottom of the glass. Pour over ice if available. The finished drink should be pale green, lightly cloudy, sour, and saline with a persistent mint and cumin finish; garnish each glass with a sprig of fresh mint.