Preheat oven to 400°F. Line two baking sheets with parchment paper and set aside.
Cut squash in half lengthwise, into two pieces. Place skin-side down on a prepared baking sheet. Brush with maple syrup and pour lemon juice over top. Sprinkle with salt and pepper. Bake for 40 minutes or until tender and caramelized.
Meanwhile, add reserved squash seeds to a small bowl with vegetable oil and chili powder. Toss to coat, then arrange in a single layer on the remaining prepared baking sheet. Place in oven (alongside roasting squash is fine) and bake for 15 minutes. Toss seeds with a spatula, and cook for 15 minutes more or until browned.
Remove squash from oven and let rest until cool enough to handle. Use a spoon to scoop out the flesh and place into a blender. Add vegetable broth and purée for 30 to 45 seconds or until smooth. Set aside.
In a small bowl, stir sour cream briskly with a spoon until smooth. Divide soup evenly among four bowls, top with a dollop of sour cream, and add a sprinkle of toasted seeds. Serve immediately.