Preheat oven to 425°F.
On a large baking sheet, toss together squash, cauliflower, onion, oil, and salt. Spread out in an even layer and roast, without stirring, until vegetables are tender and browned, about 30 minutes.
Transfer vegetables to a large saucepan and add broth, coconut milk, and curry powder. Bring to a boil over high heat, then reduce heat to simmer, partially covered, for 15 minutes. Stir occasionally.
Use a hand blender to puree soup until very smooth, or puree in batches in a normal blender.
Divide soup among 4 bowls, drizzle with coconut milk, and sprinkle with pumpkin seeds.