Spread the bread cubes on a large rimmed baking sheet.
In a small bowl, whisk together the butter, olive oil, dijon mustard, and salt. Drizzle mixture over the bread cubes and toss well to ensure bread cubes are evenly coated.
Bake for 10-15 minutes, until croutons begin to turn golden and crunchy. Halfway through the baking time, gently flip the croutons for even baking.