Heat oven to 450°F (232ºC) with racks set in upper-middle and lower-middle positions. Line two baking sheets with parchment paper.
Whisk together flour and cornstarch in a large bowl. Add cauliflower florets and toss to coat.
Dump cauliflower into a strainer set over a sink; shake to discard excess flour.
In now empty large bowl, beat 3 eggs. Add flour-coated cauliflower and stir to coat using rubber spatula.
Whisk together 2 cups Panko, 2 tablespoons oil, 1½ teaspoons garlic powder, 1¼ teaspoon salt, and 1 teaspoon pepper in a second shallow bowl.
Dredge cauliflower pieces in Panko mixture to coat.
Arrange cauliflower pieces in a single layer on prepared sheets.
Bake cauliflower until crispy and golden, 22–25 minutes, flipping pieces and rotating pans halfway through.