Preheat the oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a large bowl, whisk together flour, cornstarch, water, garlic powder, salt, and pepper to make a smooth batter.
Dip each cauliflower floret into the batter, shaking off excess, and place them on the prepared baking sheet.
Lightly brush or spray with oil, then bake for 25-30 minutes until golden and crispy, flipping halfway through.
While cauliflower bakes, prepare the sauce by combining soy sauce, rice vinegar, maple syrup, ketchup, and sesame oil in a saucepan over medium heat.
Bring to a simmer, then stir in the cornstarch slurry until sauce thickens.
Toss the baked cauliflower in the sauce until fully coated.
Garnish with toasted sesame seeds and green onions before serving.