A spiced vegetable filling pressed between buttered, cheese-melted bread until deeply crisp — finished with a cool drizzle and a scatter of chili flakes.
In a bowl, mix the mashed potato with the cabbage, carrot, peas, onion, chile, cilantro, chaat masala, cumin, and a pinch of salt.
Spread one side of each bread slice with green chutney. Divide the vegetable filling and cheese between four slices, and top with the remaining bread, chutney-side down.
Butter the outsides of the sandwiches. Toast in a hot skillet or panini press 3-4 minutes per side, pressing down, until deeply golden and crisp and the cheese has melted.
Slice each sandwich into triangles and stack on a plate.
Drizzle with the thinned sour cream, scatter with cilantro and chili flakes, and serve hot.