Meanwhile, dissolve 1/4 tsp of citric acid crystals in half cup water.
Peel two mangoes and keep aside.
Keep the flame on SIM, and add the citric acid solution a tsp at a time stirring the milk all the time.
Do this until the solids separate out leaving a greenish whey, do not add any more citric acid now.
Using a large mesh colander, filter out the solids (chenna).Press lightly to remove excess whey but do not squeeze it dry. Let this sit for 10 minutes or until cool enough to handle.
Remove the chenna onto a plate.
Add 6 tbsp of powdered sugar and knead well for five minutes until sugar is well mixed and you get a soft yet grainy dough.
In a rectangular / square pan or box, press one half of the chenna tightly into the base.
Slice the mangoes thinly and place over the chenna layer.
Do this until the whole layer is covered with mangoes.
Press the remaining chenna over the mango layer to make a sandwich.
Chill the pan / box for 30 minutes or so in the refrigerator. Do not freeze.
Remove and cut into pieces of desired sizes. Serve chilled.