Preheat oven to 425°F (220°C). Line a baking tray with parchment paper.
Mix non-dairy milk and vinegar in a small bowl and set aside to curdle.
In a large bowl, whisk together flour, cornstarch, onion powder, garlic powder, salt, and pepper.
Add the milk mixture and stir until smooth. Dip each cauliflower floret in the batter, then coat with panko breadcrumbs.
Place coated cauliflower on the baking tray. Bake for 25–30 minutes, flipping halfway through, until golden and crispy.
While baking, whisk together vegan mayo, sriracha, and agave in a bowl to make the bang bang sauce.
Once baked, toss the crispy cauliflower in the sauce or serve the sauce on the side for dipping.