In a large bowl, combine the flour, cornstarch, 1½ teaspoon salt, paprika, garlic powder, onion powder, and black pepper.
In a separate bowl, whisk together the milk, mustard, vinegar, and remaining ½ teaspoon salt.
Heat ½ inch of oil in a large high-sided pan to 350°F.
Dip each piece of cauliflower first into the milk mixture, then into the flour mixture, turning to coat all sides. Shake off any excess, then repeat by dipping each piece back in the milk mixture, then into the flour mixture. Place on a baking sheet and repeat with remaining cauliflower.
When oil is hot, carefully add 3-4 pieces of cauliflower to the pan with a slotted spoon or tongs, working in batches to avoid lowering the oil temperature or crowding the pan.Cook for 4-5 minutes, turning regularly, until crispy, browned, and fork-tender. Transfer to a paper towel-lined baking sheet and repeat with remaining cauliflower.
Serve hot with hot sauce and/or ranch dressing.