For the BBQ rub: Whisk together turbinado sugar, salt, granulated sugar, chili powder, smoked paprika, granulated garlic, onion powder, cayenne, cumin and pepper in a medium bowl. Set aside.
For the cauliflower: Heat a grill for cooking over high heat and line a baking sheet with a large doubled piece of heavy-duty foil.
Place cauliflower on prepared baking sheet. Coat entire head of cauliflower in butter using a pastry brush, then sprinkle 1/4 cup of the BBQ rub on the entire surface, including the bottom. Roll the edges of the foil up to create a rim around the cauliflower, then tent with more foil. Place baking sheet on grill and cook, covered, for 15 minutes. Carefully remove the foil lid and cook, covered, until just tender, another 25 minutes. Brush on BBQ sauce and continue to cook, covered, until nicely glazed, about 5 minutes. Cut up and serve with homemade ranch dressing, for dipping.
Homemade Ranch Dressing:
Mix the mayonnaise, buttermilk, garlic, dill, honey and mustard in a bowl. Season to taste with salt and pepper. Store in an airtight container for up to one week.