Preheat oven to 400˚F. Line a baking sheet with cooking spray and set aside.
Pour half of panko into a food processor and blend until finely ground.
Add ground panko to other half and stir together. Set aside.
Place rice flour, tapioca starch, garlic powder, onion powder, dry mustard, and salt into a large mixing bowl and whisk together.
Add buttermilk and egg and whisk together until evenly combined and no lumps remain.
Add cauliflower florets into batter and fold together until every pieces is completely coated.
Working in small batches, remove cauliflower florets from batter, shaking off as much excess batter possible. Transfer florets to panko and evenly coat.
Shake off any excess panko and transfer coated florets to prepared baking sheet, spacing each piece about 1/2” to 1” apart. Once all cauliflower florets are coated and on the baking sheet, spray with cooking spray until well coated. Season with salt and pepper to taste.
Bake florets in oven for 20 minutes or until lightly browned and tender. Remove florets from oven and cool, about 2 minutes.
Pour 1/3 sauce into a mixing bowl and add 1/3 of the florets. Gently toss florets until evenly and lightly coated. The heavier the coating, the less crispy your buffalo cauliflower will be. (Alternatively, use a pastry brush, lightly brush sauce onto each floret for a more crisp result). Repeat until all florets have been coated.
Transfer florets back to the baking sheet and bake for an additional 4 minutes.
Transfer buffalo cauliflower to a platter. Sprinkle cauliflower with sliced green onions and serve immediately with ranch or bleu cheese dressing.