For the chhenna, bring the milk to a boil over high heat. Set aside to cool slightly to 77 °C/170°F.
Mix the vinegar in one and three-fourth cups of water and add to the hot milk. Stir lightly till the milk curdles. Add three to four cups of water and a few ice cubes and stir.
Transfer the chhenna onto a worktop. Mix together half teaspoon of refined flour and the cornflour and add to the chhenna. Knead, pressing with the heel of your hand, till the mixture is smooth.
Mix the remaining refined flour with half a cup of water and set aside.
To make the syrup, cook the sugar with five cups of water, stirring continuously till all the sugar dissolves. Add the milk and let the syrup come to a boil. Collect the scum which rises to the surface with a ladle and discard. Continue to cook the syrup for a few minutes longer. Strain the syrup into a bowl.
Take one cup of the syrup, reserving the rest, in a deep, wide non-stick pan and add four to five cups of water. When the syrup begins to boil add the chhenna balls. Add half the flour mixture. The syrup will froth. Cook, gently agitating the syrup so that the balls do not stick to the bottom of the pan.
Slowly drizzle half a cup of water along the sides of the pan every five minutes so that the syrup does not thicken. Continue cooking for fifteen minutes or till the chhenna patties spring back when pressed. This is a sign that they are cooked.
Remove the balls with a slotted spoon and place in the reserved syrup. Chill for at least two hours so the rosogulla absorb the syrup. Serve.