Vada Pav (Potato Fritter Sandwich) From Maneet Chauhan & Jody Eddy

Name: Vada Pav (Potato Fritter Sandwich) From Maneet Chauhan & Jody Eddy
Cuisine: Indian
Ingredients: 3 russet potatoes; 2 tablespoon vegetable oil, plus more for frying; 1 teaspoon brown mustard seeds; 1 teaspoon cumin seeds; 1 (1-inch) piece fresh ginger, peeled with a spoon and finely chopped; 2 garlic cloves, finely chopped; 4 fresh curry leaves; 1 teaspoon ground turmeric; 1/2 teaspoon hing (asafetida); 1/2 teaspoon Kashmiri or other red chili powder; Kosher salt; 1/2 cup lightly packed finely chopped cilantro leaves; 2 teaspoon chaat masala; Fresh lime juice; 1 cup chickpea flour (besan); 1 teaspoon ground coriander; 1 tablespoon unsalted butter; 4 flaky white buns, such as potato or brioche buns; Green chutney and coconut chutney, for serving; Pan-fried serrano chiles, for serving (optional)