In a mixing bowl add mashed potato, finely chopped onion, coriander leaves, green chilli, black salt, salt, chaat masala and red chilli powder (optional).
Mix everything well together. Set aside.
Make sure the potatoes are not over-boiled (See tips above).
Apply butter on one bread and mint-coriander chutney on another.
Spread aloo (potato) filling over the bread.
Place one bread over the other, making sandwich.
Apply butter on one side.
Place the buttered side of sandwich on a heated tawa or grill pan.
Apply butter on the other side.
Cook on both the sides on a low flame till crisp and brown. Press the sandwich gently using spatula for perfect cooking.
Serve hot with ketchup and dip of your choice.