In a blender, combine the cashews, ginger, garlic, jalapeno, mustard seeds, garam masala, and turmeric and pulse until the ginger and garlic are finely chopped. Add the mango nectar and lemon juice and puree until smooth, scraping down the side of the bowl. Add the yogurt and 1 tablespoon of salt and pulse to blend. Pour the mixture into a large bowl. Add the shrimp and toss to coat. Refrigerate for 2 hours, stirring once or twice.

Light a grill. Thread the shrimp on pairs of skewers, being sure to leave on some of the marinade. Brush with the melted butter and sprinkle with salt. Oil the grate and grill the shrimp over high heat, turning occasionally, until lightly charred and cooked through, about 8 minutes. Serve the shrimp with the Cilantro and Yogurt Sauce.