Cut cauliflower into large florets. In large bowl, toss florets with oil until well coated. In another bowl, stir together cornstarch, chili powder, salt, pepper, garlic powder, onion powder and brown sugar. Sprinkle spice mixture over cauliflower; toss to coat.
Grill cauliflower on medium heat 10 to 15 min., turning occasionally, until lightly charred and tender. Meanwhile, put BBQ sauce in large, clean bowl. Add grilled cauliflower; toss to coat.
Serve with ranch dressing for dipping.