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.
Alternatively, bake cauliflower bites on parchment paper-lined baking sheets in 425˚F (220˚C) oven 15 to 20 min., turning occasionally, until and golden brown and tender, then toss with BBQ sauce.