Dump beans, onion, tomatoes, potato and green chili in a large mixing bowl.
Sprinkle chaat masal and garam masala. Toss.
To make tamarind chutney, add tamarind pulp, sugar and 1 cup of water in a saucepan and let this come to a boil. Continue cooking for 10-12 minutes or until the pulp breaks down. Use the back of the spoon to help the process.
Next, strain the tamarind liquid into a bowl. Make sure to press down on the pulp with a spoon to extract as much juices as you can. Discard the pulp (I chew on it :p).
Now pour the liquid back into the saucepan and cook for another 5 minutes. The chutney will be very liquidy.
Remove from stove and let the chutney cool. As it cools, it will thicken.
Pour the cooled chutney into the chaat and toss.
Garnish with fresh chopped cilantro and serve.