Take milk in any utensil and place it on high flame. Allow the milk to boil. When milk starts simmering, reduce the flame and stir constantly until it reduces half in quantity (its very important to stir the milk constantly so that its doesn't stick to the bottom of pan).
When milk gets thick and half in quantity, turn off the flame. Add half the amount of sugar and cardamom powder into it. Caramelize rest of the sugar.
To caramelize sugar, take sugar in a utensil with thick and heavy base. Keep stirring constantly and cook on medium flame. Within few minutes sugar will start melting. Keep stirring the sugar constantly. When sugar melts completely and is caramelized, add it in milk and mix well.
To set the sugar mixed milk, take a small terracotta or clay pot and add milk into it. Milk should neither be too hot nor too cold. It should be medium hot. Add 2 tsp curd in clay pot filled with milk. Cover and place it on a warm place to set (Any other utensil can be used to set mishti doi).
It takes 10-12 hours to set the milk. Once milk is set, mishti doi is ready. Serve chilled and delectable mishti doi and relish eating.