Melt the butter in a large wide pot or medium Dutch oven over medium heat. Add the ginger, garlic, onion, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until the onion is tender, about 8 minutes. Add the cauliflower and curry powder and continue to cook, stirring occasionally until the cauliflower is well coated in spice and slightly softened, about 5 minutes more. Add the broth and 2 1/2 cups water; bring to a boil. Reduce the heat to maintain a simmer and cook until the cauliflower is very tender, about 20 minutes. Remove from the heat and cool slightly.
Puree the soup in the pot with an immersion blender or, working in batches, transfer the soup to a blender and puree until smooth, being careful not to fill the carafe more than half full per batch (keep the lid cracked to allow steam to escape). Rewarm the soup over medium heat and adjust the seasoning with additional salt and pepper. Ladle into 4 bowls and sprinkle with some cilantro and chopped cashews.
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