In a medium bowl, combine the hung curd, mayonnaise, finely chopped green chilies, fresh coriander, black pepper, and salt. Mix well until smooth and creamy, with no lumps remaining.
Take a slice of crustless bread and spread a generous layer of spicy cilantro dip evenly over it. Top with a generous layer of the prepared curd-mayo spread.
Arrange 8–9 cucumber slices closely over the creamy layer to create a fresh, crunchy bite in every mouthful.
Sprinkle the cucumber with red chili flakes and black salt for an extra burst of flavor.
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.
Using a sharp knife, cut the sandwich into triangles or squares, depending on your preference. For clean, neat cuts, wipe the knife with a clean cloth or paper towel between slices.