Rinse the cauliflower and cut into florets if not already done; pat the florets dry with paper towels.
In a large bowl, whisk together 1 cup all-purpose flour, 2 tablespoons cornstarch, 1 teaspoon baking powder, 1/2 teaspoon salt, 1/2 teaspoon garlic powder, and 1/4 teaspoon black pepper until evenly combined.
Gradually add 1 cup water to the dry ingredients while whisking, until the batter is smooth and free of lumps and thick enough to coat the florets.
Heat vegetable oil in a deep-fryer or a large, heavy pot to 365°F.
Working in batches so you do not overcrowd the oil, dip cauliflower florets into the batter, coating each piece evenly and allowing any excess batter to drip back into the bowl.
Carefully add the battered florets to the hot oil and fry until golden and crispy, about 3–4 minutes per batch.
Use a slotted spoon or tongs to transfer the fried cauliflower to paper towels to drain. Repeat with remaining florets, allowing the oil to return to 365°F between batches if needed.
Serve the fried cauliflower hot.