In a bowl, combine the shrimp with all of the ingredients for the marinade; let marinate while you make the butter sauce.

Heat butter or ghee a pan/pot. Fry the onions until they start to sweat (about 6 minutes) while scraping up any browned bits stuck on the bottom of the pan.

Add garlic and ginger and sauté for 1 minute until fragrant, then add ground coriander, cumin and garam masala. Let cook for about 20 seconds until fragrant, while stirring occasionally.

Add crushed tomatoes, chili powder and salt. Let simmer for about 10-15 minutes, stirring occasionally until sauce thickens and becomes a deep brown red color.

Remove from heat, scoop mixture into a blender and blend until smooth. You may need to add a couple tablespoons of water to help it blend (up to 1/4 cup). Work in batches depending on the size of your blender.
(I use a hand blender directly in the pot I'm cooking the butter sauce in.)

Pour the puréed sauce back into the pan. Stir the cream, sugar and crushed kasoori methi (or fenugreek leaves) through the sauce. Add the shrimp with all the marinade into the pan/pot and cook until the shrimp is cooked through and the sauce is thick and bubbling.

Garnish with chopped cilantro.