Preheat the oven to 425 degrees F (220 degrees C).

Place butternut squash in a baking dish, flesh-sides up. Brush 1 tablespoon melted butter over flesh; season with 1/2 teaspoon salt, 1/2 teaspoon pumpkin pie spice, and cayenne pepper.

Roast in the preheated oven until tender, about 1 hour. Cool for 15 minutes.

Place a large stockpot over medium heat; stir in 1 tablespoon butter until melted. Add onion; cook and stir for 2 minutes. Add curry powder; cook and stir for 1 minute. Stir in coconut milk; bring to a boil.

Scoop flesh from butternut squash; stir into coconut milk mixture. Stir in 2 1/2 cups vegetable stock; season with remaining 1/2 teaspoon salt, remaining 1/4 teaspoon pumpkin pie spice, and 1/4 teaspoon nutmeg. Bring to a boil, reduce heat to low, and simmer until soup is heated through.

Blend soup using an immersion blender on low speed. Simmer until soup is smooth and thickened, about 20 minutes more. Add more vegetable stock for a thinner consistency; season with salt.

Divide soup among bowls; top servings with pumpkin seeds and 1 pinch nutmeg.