Preheat the oven to 450°F/230°C.
Using a knife, cut the cauliflower into florets and discard the leaves and the stems.
In a large bowl, combine the flour, cornstarch, oat milk, lemon juice, 1 teaspoon of garlic powder, ½ teaspoon onion powder, salt, and black pepper. Stir to combine. The batter should look like sticky tempura batter. If the batter is too thick, dilute it with a little non-dairy milk.
In another bowl, place the Panko breadcrumbs and season with the remaining garlic powder and onion powder. Season with salt and pepper.
Transfer the cauliflower florets to the tempura batter to coat them completely, then the Panko breadcrumbs. This can be done a second time.
Place the breaded cauliflower on a parchment-lined baking sheet, leaving enough space between each floret. Bake for 22-25 minutes, turning them half-time through. Remove from the oven.
Combine all sauce ingredients in a small bowl. It should be creamy and not too liquid. Put aside.
Using a spatula, toss the baked cauliflower into the sauce, then using tongs or a spoon, remove and return to the baking sheet. Cook for another 2 to 3 minutes so that the sauce penetrates the bites.
Remove from the oven, optionally sprinkle with sesame seeds, or garnish with cilantro or chopped green onion.
This Bang Bang cauliflower does not keep well and is best enjoyed the same day.