Rice flour is the key ingredient for making bhaji batter crispier. Adding a tablespoon or two to your regular batter mix will significantly improve the texture.
The crispiness of bhaji batter depends on how quickly the moisture evaporates during frying. Rice flour contains less gluten than wheat flour. Gluten develops when wheat flour is mixed with water, creating a stretchy network. This network can trap moisture, leading to a softer, less crispy bhaji. Rice flour, on the other hand, absorbs less water and doesn't form gluten. This allows the moisture to escape more readily during frying, resulting in a crispier texture. You can substitute a portion of your regular flour (besan or gram flour is common) with rice flour, or simply add a tablespoon or two to your existing batter recipe. Experiment to find the perfect ratio for your taste. Other ingredients that can contribute to crispiness include cornstarch or semolina (rava).
Don't overmix your bhaji batter! Overmixing develops gluten, even in besan, leading to a tougher, less crispy bhaji. Mix just until the ingredients are combined, and let the batter rest for a few minutes before frying.