In a medium pot, heat the oil over medium-high heat. Add the onions and garlic and cook, stirring, until soft, about 3 minutes. Add the curry powder, salt, and cayenne, stir, and cook until fragrant, about 30 seconds. Add the zucchini, reduce to medium heat, and cook, stirring occasionally, until soft, 5 to 6 minutes. Add the stock and bring to a boil. Reduce the heat and simmer until the zucchini is very tender, about 20 minutes.
Remove from the heat. With a hand-held immersion blender, or in batches in a food processor, puree the soup. Return to medium heat and stir in the cream. Simmer for 3 minutes. Adjust the seasoning, to taste.
Pour into a clean container and let cool slightly. Refrigerate until well chilled, 4 to 6 hours.
To serve, ladle into large cups, garnish with cilantro, and serve with hot pappadums or pita toast. This soup may be served hot or cold.