Adjust the amount of jaggery, salt, tamarind, chili powder and ginger powder according to personal taste.
To make roasted cumin powder, dry roast some cumin seeds on medium heat until slightly golden brown, cool and powder.
To store the sauce for a longer time, cook on medium heat for about 10 minutes until slightly thick, cool and refrigerate. Add bananas just before serving.
If making ahead, make the sauce and refrigerate; just before serving, add bananas and raisins.