Preheat oven to 450°F. Coat a baking sheet with the olive oil.
Whisk the flour, baking powder, and salt together in a large bowl. Whisk in the milk until a smooth batter forms.
Add the cauliflower florets to the bowl of batter and toss until coated. Remove individual florets with a slotted spoon, shaking off excess batter before placing on the prepared baking sheet in a single layer.
Bake in the oven for 20 minutes (or until golden brown), flipping them halfway through.
While cauliflower is baking, stir the soy sauce, maple syrup, rice vinegar, ginger, garlic, sesame oil and chili flakes together in a small saucepan. Bring to a boil then reduce to a simmer. Simmer for 5 minutes.
Stir the cornstarch together with 2 tablespoons cold water in a small bowl. Pour the mixture into the sauce and cook another minute or two until thickened. Remove from heat.
Pour the sauce over the cauliflower and toss to combine (alternatively you can serve the sauce on the side).
Return the pan to the oven and cook another few minutes until the sauce starts to caramelize. Remove from oven. Serve the cauliflower garnished with scallions, if desired.