In a medium-sized pot, melt butter over mediumlow heat. Add shrimp shells and cook until the shells start to brown slightly. Add water and lemongrass. Bring to a simmer and cook for 20-25 minutes. Turn off heat and allow to steep for another 15 to 20 minutes. Strain through a fine-mesh strainer into a bowl or jar. Shrimp stock can be kept in the refrigerator for 3 to 4 days.
In a medium-sized pot, over medium heat, add oil, onions, celery and carrots and cook for 8 to 10 minutes or until soft. Add rice, lime or bay leaves and tomato paste. Add shrimp stock (4 cups). Simmer for 20 to 25 minutes or until all of the vegetables and rice are fully cooked.
Once all ingredients are soft, transfer to a blender or food processor and blend until smooth. Strain through a fine-mesh strainer into a medium pot. Add coconut milk and season with salt and pepper. Bring back up to a boil. To serve, divide into 4 bowls and garnish each with a freshly cooked shrimp, cilantro and chives.