1. Prepare pumpkin seeds: In a bowl, stir together cayenne, sugar and salt. Melt butter in a skillet over medium heat. When butter sizzles, add pumpkin seeds and cook, stirring, 3 minutes. Sprinkle in spice mixture and continue to cook, stirring, until seeds are toasted, 2 to 3 minutes longer. Remove to a bowl and let cool.
2. Make soup: Melt butter in a large pot over medium heat. Add onion, apples and ginger and sauté until tender, about 8 minutes. Sprinkle flour, cumin and curry and chili powders over onion mixture and stir for 1 minute (mixture will be dry).
3. Gradually whisk in broth and cook, stirring occasionally, until mixture begins to thicken, about 6 minutes. Whisk in pumpkin and coconut milk. Season with salt and pepper. Bring to a low boil and cook for 5 minutes, stirring often. Reduce heat and simmer for 10 minutes. Remove from heat and let cool slightly.
4. Working in batches, pour soup into blender and blend until smooth. Return to pot to rewarm. Serve hot, topped with yogurt, if desired, and sprinkled with pumpkin seeds.