In a bowl beat the yogurt along with 1/2 cup of water and keep aside.
In another small bowl mix together chilli powder, turmeric powder, cumin powder and coriander powder with 1/4 cup of water and keep it aside.
Wash and peel the pointed gourd diagonally and alternatively (as shown in the picture), and prick it with a fork. Marinate it with little salt and turmeric and keep it aside for 5 minutes.
Heat 2 tbsp of oil in a large pan on medium heat. When hot fry the parwal until it’s soft and golden brown. Drain and keep aside.
Add the rest of the oil in the same pan or kadhai on high heat until it’s smoking hot. Then reduce the heat to medium low. Add cinnamon, cardamom, cloves and bay leaf and fry for few seconds.
When the aroma arises from the spices add the ginger paste and green chilies and fry for 1 to 2 minutes stirring frequently.
Now pour in the masala paste and sauté 1-2 minutes.
When you see the oil seeping out from the masala, lower the heat to the lowest setting.
Now add 2 tbsp of yogurt stirring continuously. This will slowly reduce the cooking temperature and this will avoid the yogurt from curdling. Add 2 more tbsps of yogurt stirring continuously. Then slowly pour in the yogurt and make sure you stir continuously.
Now add salt and sugar and mix.. Cover and cook on lowest heat for 5-6 minutes. Stirring intermittently.
Then slowly add in the fried parwal/potol and mix it well with the yogurt sauce. Check the seasoning add salt/ sugar if required.
Cover and cook for 8-10 minutes until the parwal is cooked through and oil see through the gravy. Finally stir in the bengali garam masala powder and cook for further 1 minute and take it off the heat.
Finally sprinkle with chopped coriander leaves and serve it hot with rice or roti.