Melt the butter in a 14-inch saute pan over medium-high heat. Add the mustard seeds and cumin seeds and toast until fragrant, about 1 minute. Add the onion, chiles, ginger, and garlic and saute, stirring occasionally, until the onion is translucent, about 3 minutes. Add the diced tomatoes and their juices, and cook until the liquid is reduced by half, 3 to 4 minutes. Raise the heat to high and add the coconut milk. Simmer until the milk is reduced by half, 3 to 4 minutes.
Season the shrimp with 1 1/2 teaspoons of the salt. Add the shrimp to the pan and cook, stirring as needed, until they are curled, pink, and just cooked through, about 3 minutes.
Remove the pan from the heat. Season the shrimp with the remaining 1/2 teaspoon salt, and garnish with the cilantro and chives, if desired. Serve immediately over steamed basmati rice.