While the corn is being prepared, finely chop the onion, tomato, green chili, and fresh coriander and set them aside in separate small bowls.

Pour the water into a saucepan, stir in the salt, and bring it to a rolling boil over high heat.

Add the sweet corn kernels to the boiling water and cook until they turn bright yellow and tender to bite.

Drain the boiled corn kernels thoroughly in a colander, shaking off any excess water so the chaat does not turn soggy.

Let the drained corn kernels sit uncovered for a few minutes to cool from boiling-hot down to a pleasantly warm temperature before mixing in the other ingredients.

Transfer the warm corn to a large mixing bowl and add the chopped onion, tomato, and green chili, tossing gently to combine.

Sprinkle in the chaat masala, red chili powder, and roasted cumin powder, then toss everything together until the corn is evenly coated in the spices.

Squeeze the fresh lime juice over the chaat, drizzle in the melted butter if using, and fold in the chopped coriander just before serving warm.