In a large bowl, whisk together flour, cornstarch, and baking powder. Add ice water, egg, and 1 teaspoon salt, and stir to combine. The batter should be thin.
In a large skillet over medium heat, heat about ¼” vegetable oil until oil is shimmering. Toss cauliflower in batter until fully coated. Working in batches, add cauliflower, and cook until golden on all sides, about 4 minutes per side. Drain on paper towels and season with more salt, if necessary.
In a medium bowl, whisk together garlic-chili sauce, soy sauce, brown sugar, sesame oil, and ginger. Toss fried cauliflower in sauce. Sprinkle with sesame seeds and green onions and serve.