In a medium saucepan, melt the butter over medium heat. Add the leeks and cook until wilted, about 5 minutes. Add the garlic, ginger and curry powder and cook, stirring, until fragrant, about 1 minute. Add 2 cups of the popcorn and toss to coat in the butter.
Add the stock, pumpkin and heavy cream. Bring to a simmer. Cook until the leeks are very tender, the popcorn has dissolved away from the kernels and the kernels are softened, about 15 minutes.
Puree the soup with a hand blender or in batches in a countertop blender. Stir in the lime juice and taste for seasoning, adding a little salt if necessary. Thin with up to 1/2 cup water, if desired. Serve in individual bowls with the remaining popcorn as a garnish.