Chop the head of cauliflower into bite-sized florets. Rinse under cold water and drain well.
In a large mixing bowl, combine all-purpose flour, garlic powder, black pepper, and a pinch of salt. Gradually whisk in water until smooth and thick enough to coat the cauliflower.
Heat 1 to 2 inches of vegetable oil in a frying pan or deep fryer over medium heat. Test the oil by dropping some batter in; it should sizzle and rise to the surface.
Dip each cauliflower floret into the batter, coating fully. Let excess batter drip off before frying.
Carefully place battered cauliflower into hot oil without overcrowding. Fry for 4-5 minutes, turning occasionally, until golden brown and crispy.
Remove fried cauliflower with a slotted spoon and place on paper towels to drain excess oil. Let cool slightly.
In a small saucepan, combine sweet chili sauce, sugar, soy sauce, and rice vinegar. Heat over medium heat, stirring until sugar dissolves and sauce is warmed through.
Drizzle warm sweet chili sauce over fried cauliflower. Toss gently to coat and serve immediately while hot and crispy.