Preheat oven to 475 degrees F. To a rimmed baking sheet, add the shallots, drizzle with 1 tablespoon melted butter and season with ¼ teaspoon salt; spread in a single layer. Roast until softened and starting to brown, about 5 minutes.
Meanwhile, butterfly the shrimp by making a deep cut down the center of the shrimp's back without cutting all the way through; transfer to a medium bowl.
Combine the remaining 5 tablespoons melted butter, ginger, garlic, curry powder and garam masala in a separate large bowl. Add 2 tablespoons of the spice butter to the shrimp; add the paprika and toss.
To the baking sheet, add the rice, remaining ½ teaspoon salt, and ½ cup chopped cilantro to the remaining spice butter; toss. Add the shrimp to the baking sheet and spread in a single layer. Drizzle rice with ¼ cup water. Cover the pan tightly with foil and bake until the rice is hot and the shrimp are cooked through, about 18 to 20 minutes.
After baking, remove the foil and top with remaining ½ cup chopped cilantro and roasted cashews. Serve hot.