Heat the milk in a wok on medium flame. Once the milk starts boiling, lower the flame and add 2 tbsp either of lime juice or vinegar to it. Stir gently.
The milk will curdle and greenish whey separating out.
Pour the curdled milk over a muslin cloth lined colander. Drain the whey.
Gather the sides of the muslin cloth. Chenna is collected inside it. Squeeze out whey gently and rinse the chenna in running water to get rid of after taste and smell of lemon or vinegar.
Squeeze out the liquid by gently pressing the collected bundle of chenna in the muslin cloth and keep it under some weight for 15-20 minutes.
Take out the chenna to a plate or wide bowl . Add powdered sugar and green cardamom powder to it. Mix well and knead it like a dough for 3-4 minutes.
Transfer the prepared mixture to a heavy bottom non stick wok or pan. Cook the mixture on a low flame for about 5-7 minutes while continuously stirring. The resultant chena and sugar mixture will dry out yet will retain moisture.
Put off the flame and allow the mixture to cool down enough to handle. Knead it well , while it is still warm. Divide the mixture into medium sized portions and make round ball .
Using your palms gently press it to flatten slightly. For garnishing, place sliced Pistachio on the center of each Sandesh and press it lightly with your thumb.
Refrigerate the Sandesh at least for 1-2 hours before serving. Sandesh tastes best when served chilled. Consume it within 2 days.