Set a small strainer on top of a bowl (make sure that the strainer is not touching the bottom of bowl). Add yogurt to the strainer and set aside for 30 minutes. Discard the liquid collected at the bottom of bowl after 30 minutes. Yogurt culture is ready to use.
Add milk to a heavy bottom pot and set it on a low medium stove. Let the milk come to a boil and reduce it for 8-10 minutes. Don't thicken too much else yogurt wont set. Take off the stove once reduced and let stand nearby.
Add sugar to a wide pan or wide base kadai. Make sure that the sugar is not piled up in one place. Gently spread it in a single layer on the pan. Place the pan or kadai on low flame.
Now let the sugar melt. Dont stir it or touch it else it will crystalize. The sugar will go through stages of melting to a liquid to changing to light brown color to eventually turning golden brown color. This entire process can take somewhere between 6-8 minutes. Dont let the sugar syrup turn dark brown else the mishti doi will taste bitter.
Slowly start ladling milk into the caramalized sugar while stirring. There will be a lot of bubbling initially as soon as you add milk, be careful. Once the bubbling settles, add in all of the milk. Also mix in the condensed milk.
Let the milk mixture cool down. The milk mixture should be warm- you should be able to feel the warmth when you dip a finger into it.
In a large bowl, whisk the yogurt culture until smooth. Add the milk mixture and combine the two nicely until all the yogurt has dissolved.
Pour the curd mixture into earthern pots or any bowl you want to set the mishti doi in. Cover the top with lids or aluminum foil.Note- If you plan to set the yogurt in Instant pot, make sure that the bowls are able to fit in the steel insert.
Keep the mishti doi in a warm place to set. You could wrap it in kitchen towels to insulate. It could take between 6-12 hours or longer for mishti doi to set. Don't be afraid and give hope- as long as you are able to maintain the temperature, it will set. Using Instant Pot for setting mishti doi - I use Instant Pot Yogurt function. Carefully stack the earthern bowls inside the steel insert of IP. If you want, you can place the bowl on top of rack that comes with IP. Close the lid and press the yogurt button. I set it to 8 hours.
Once the mishti doi is set, it will be still quite delicate. Carefully place in the refrigerator and let chill for 4-8 hours.
Storage - Store set mishti doi for upto a week refrigerated. Serve chilled as a delightful dessert after meals. You can enjoy on its own or garnish with chopped nuts, such as pistachios or almonds, to add a crunchy texture.
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Notes
Choose full fat milk for creamy and rich mishti doi. Skimmed milk won't give the same texture and taste.
Dont reduce the milk too much, else it wont set. Boil only for few minutes.
Yogurt Starter - Make sure to choose yogurt or curd containing active live cultures as a starter. You can use store-bought yogurt or a small portion of a homemade dahi (curd). Make sure the yogurt you use is fresh.
Dont use lot of condensed milk - It can make your mishti doi sticky or interfere with the setting. You may completely skip condensed milk if you wish.
Add flavors like ground cardamom if you wish.
Caramalize the sugar without stirring on low heat. Dont let the sugar caramelize to very dark shade, it will be bitter and your mishti doi will have a burnt taste.
You could add desired amount of sugar to the milk. Keep in mind that the yogurt cultures will consume some of the sugar during fermentation, so it is better to keep in on sweeter side.
If possible, choose earthen pots for setting mishti doi for the classic taste and texture.
Setting Time - Depending on the ambient temperature, mishti doi could set from 6 -12 hours. It tastes longer to set than regular curd. During summers, it will set quickly as compared to winter months. Keep and eye while its setting and you could leave it to ferment longer if you would like it to be extra tangy.
Dont skip refrigeration. It thickens and sets the yogurt and brings out the texture of mishti doi.