Peel, seed and dice the cucumber. Place nonreactive soup pot over medium heat and add the butter, onions and mustard seeds. Sauté for 2 minutes. Add broth, coconut milk and bay leaf. Bring to a boil and let simmer over low heat for 15 minutes. Remove bay leaf. Make an ice bath with water and ice in a large bowl. Place soup into a smaller bowl and immerse into the ice bath. Stir well to cool down and chill completely. When soup is chilled, add dill, cucumbers and buttermilk. Season with kosher salt and cracked black pepper.