First of all heat a heavy bottom pan and pour 2 litres of milk in it. The reason why I always use heavy bottom pan and medium flame is while making anything with milk it saves milk from sticking at bottom.
After the first boil comes, switch off the flame and keep it for 10 minutes to cool down.
Now add vinegar to the milk and keep stirring.
Once milk curdles completely (it will take 1-2 minutes), add 8-10 cubes of ice to it. This will help in cooling down the temperature of milk and curdled milk will combine together to form big chunks.
Strain this on a strainer lined with Muslin cloth. You can keep the strained water (whey) to add in your curries or to knead flour as it is a very good source of protein.
Now wash this curdled milk with the running water under tap for 8-10 minutes. This helps in removing the sour taste due to vinegar.
Gather the Muslin cloth from all the corners and squeeze the extra water gently. Don’t squeeze hard otherwise the curdled milk will also come out with water.
Now hang this for 30 minutes so that extra water present will also drop down.
After 30 minutes, take out the curdled milk onto a plate. Your chenna is ready. This would be little granular and dry in appearance.
Soak the pistachios in luke warm water. Keeping aside a few, grind rest of pistachio to a fine paste.