Cut 1 head cauliflower into florets and pat them dry with paper towels to remove excess moisture.
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.
Gradually add 1 cup water to the dry ingredients, whisking until the batter is smooth and free of lumps (it should coat a spoon and be pourable but not watery).
Pour enough vegetable oil into a deep-fryer or a large, heavy pot so the florets can be mostly submerged (about 2–3 inches of oil); heat the oil to 365°F.
Working in batches, dip each cauliflower floret into the batter, letting any excess drip back into the bowl.
Carefully lower battered florets into the hot oil, avoiding overcrowding the pot. Fry each batch for about 3–4 minutes, turning occasionally, until the cauliflower is golden brown and crispy.
Use a slotted spoon or spider to transfer the fried cauliflower to a plate lined with paper towels to drain.
Repeat with remaining florets, maintaining the oil temperature at 365°F between batches.
Serve the fried cauliflower hot.