Place coriander (cilantro) leaves, green chilies, fresh ginger, chaat masala, ground cumin, and lemon juice in the bowl of a food processor. Season with salt. Process to a smooth paste. Add a little water if needed.
Green chutney sandwich:
Spread butter on one side of each bread slice. Add a layer of coriander chutney to 5 bread slices. Cover with the remaining slices.