Preheat the oven to 375°F. Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
In a small skillet over medium heat, melt the butter with the olive oil. Add the minced garlic and chopped rosemary. Cook for 1 more minute.
Pour the butter mixture over the cubed bread. Add salt and toss everything together with a large spoon or your hands until all the bread cubes are evenly coated.
Spread the coated bread cubes onto the prepared baking sheet in a single layer, making sure they aren’t overcrowded.
Bake for 15–20 minutes, flipping the croutons halfway through, until golden brown and crisp.
Remove from the oven to cool slightly. Garnish with parmesan, if desired.