Preheat oven to 425°F with a rack in the center. Wash and thoroughly dry the cauliflower and cut into large florets.
In a small bowl, combine the two tablespoons olive oil, chili powder, ¼ teaspoon of the salt and some freshly ground black pepper. Place the cauliflower florets on a baking sheet and toss well with the oil mixture. Roast for 25 minutes, turning once with a spatula mid-roasting.
While the cauliflower roasts, combine the extra-virgin olive oil, lime juice, vinegar, cilantro, garlic, and remaining ¼ teaspoon salt in a blender. Blend until smooth, stopping to scrape down sides as necessary.
Drizzle the roasted cauliflower with the dressing and serve immediately.
