Rinse a 3-quart heavy-bottomed saucepan and leave a little water in it. Add the milk and bring to a full boil over medium heat for 10 to 12 minutes.
Add ground pistachios and cardamom. Simmer on medium heat for 5 minutes.
Add sugar and cook 10 to 12 minutes more, until reduced to about 2 to 2½ cups. Keep it pourable. Transfer to a bowl to cool slightly.
Make the chenna
Set a strainer lined with muslin cloth over a large bowl.
Bring the milk to a full boil on medium heat for 10 to 12 minutes. Turn off the heat and let it rest for 3 to 4 minutes.
Mix vinegar with water. Add to the milk 1 tablespoon at a time, stirring gently, until the whey turns clear and greenish.
Do not add vinegar directly to the milk; diluting it keeps the chenna soft.
Pour into the muslin cloth. Rinse with ice-cold water to remove the sour taste.
Squeeze gently for 10 to 30 seconds, just until it stops dripping. Return to the strainer, place about 3 pounds of weight on top, and rest for 10 minutes.
The chenna should be moist but not sticky. Over-squeezing makes hard rasmalai.
Knead and shape
Place a parat on a silicone mat or damp towel. Crumble the chenna and knead with the heel of your palm for 8 to 10 minutes until smooth. Stop if it turns greasy.
Add the flour and knead lightly for 1 minute. Rest 5 minutes.
Meanwhile, bring the sugar, water, and cardamom pods to a full boil in a wide 4-quart pan over high heat, about 10 minutes.
Divide the chenna into 5 portions, each portion into 10, then halve each piece. Roll into smooth, crack-free balls, about 100 in all.
Cook and soak
Add the balls to the boiling syrup. Cook uncovered for 1 minute without stirring.
Cover and cook for 4 minutes on medium-high heat. Do not open the lid.
Turn off the heat and transfer the balls to a bowl of ice water. They should sink. Cool for 5 minutes.
Gently squeeze each ball between your palms and add to the lukewarm ras. Warm the ras for about a minute in the microwave if it has cooled.
Never add the cooked balls to cold or room temperature ras.
Refrigerate at least 2 hours before serving.
Store refrigerated in an airtight container for up to 3 days.