Preheat oven to 375 degrees F (190 degrees C). Line a baking sheet with aluminum foil.
Place bread cubes in a large bowl; drizzle butter over cubes. Toss to evenly coat. Sprinkle half the Cajun seasoning, half the Parmesan cheese, and half the onion powder over cubes; toss to coat. Spread coated bread cubes onto the prepared baking sheet; sprinkle remaining Cajun seasoning, Parmesan cheese, and onion powder onto cubes.
Bake in the preheated oven for 7 minutes; stir and continue baking until crisp, about 8 more minutes.