In a medium bowl, combine the hung curd, mayonnaise, finely chopped green chilies, fresh coriander, black pepper, and salt. Mix until smooth and creamy.
Take a slice of crustless bread and spread the spicy cilantro dip evenly over it. Top with the curd-mayo spread prepared above.
Arrange 8–9 cucumber slices over the spread.
Sprinkle red chili flakes and black salt over the cucumber.
Spread a generous layer of butter over another slice of crustless bread and place it butter-side down over the cucumber layer. Gently press the sandwich together.
Cut the sandwich into triangles or squares using a sharp knife. Wipe the knife with a clean cloth or paper towel between cuts.