Fire up your grill to medium heat, about 350°F (180°C). Lightly oil a grill pan. Set the grill pan on the grill so it warms up.
Cut the cauliflower into fourths. Remove the rib from the florets, leaving as much of the stem as you would like, making it easier to pick up.
In a large bowl, combine the flour, cornstarch, onion powder, salt, and pepper
Coat the cauliflower pieces in the milk, then coat them in the dry mixture
Place the cauliflower on the grill pan and grill for 10 minutes, then flip and grill for another 5-10 minutes. Continue grilling until the cauliflower is slightly charred and tender.
To make the BBQ sauce, combine all ingredients in a bowl. Taste and adjust seasonings as necessary.
Transfer cauliflower to a large bowl and add the BBQ sauce. Coat to combine, then serve warm.