Preheat oven to 425°F and place a wire rack on a rimmed sheet pan; spray the rack lightly with oil.
Whisk flour, water, garlic powder, paprika, salt, and pepper until a smooth batter coats the back of a spoon; fold in panko if using.
Toss cauliflower florets in the batter until evenly coated, shake off excess, and arrange in a single layer on the wire rack.
Bake for 20–25 minutes until the batter is set and edges are golden brown and slightly blistered; florets should be fork-tender when pierced.
Mix hot sauce, melted butter, and brown sugar in a bowl off heat until glossy; quickly toss baked florets in the sauce to coat.
Return sauced florets to the rack and bake 5–8 minutes at 425°F until the sauce glazes and edges firm; serve immediately with celery and dressing.