In a soup pot over medium heat, melt the butter then add the leeks and garlic. Cook, stirring often until the leeks are softened (approximately 2 to 3 minutes)
To the leeks and garlic, add the curry and cayenne powders. Cook stirring often for 1 minute.
Add the chicken stock and potatoes, then increase the heat to medium-high. Bring the stock to a boil, then lower the heat and simmer the stock for 10 to 12 minutes (until the potatoes are softened.
Using a handheld immersion blender, process the soup until smooth (or use a blender if no immersion blender or handheld food processor is available).
Add the heavy cream and stir to combine.
Add salt and pepper to taste.
Plate the soup, then top with cilantro (optional) as a garnish.