Dice out the mango flesh and slice the chestnuts. If your blender has an ice crusher setting, crush the ice cubes first, then add all the other ingredients except the cardamom. Otherwise just add everything all at once.
Blend all for about 15 seconds, then puree on highest speed for another 30, until everything is as smooth as it's going to get. Pour out into glasses and top with a sprinkle of cardamom. Serve immediately.
Buttermilk generally makes a smoother, more drinkable and lower-calorie lassi than yogurt. You can tell this is a pandemic recipe. I still had these nice peeled, roasted and vacuum-packed chestnuts leftover from the holidays, and was looking to add some umami to a liquid lunch. Now I just buy chestnuts. Besides making a potent lassi, they're good in vegeburgers, various stuffings (of course) and rice dishes.
The cardamom is important! I"m not sure you can call it a lassi without some cardamom floating on top. I'm showing this with way too much freshly-pestled cardamom, so you get the idea. Too much is just enough. Freshly smashed, it has a kind of turpentine tang that slows you down, and makes you pay attention.
Servings 0