Preheat the oven to 375 degrees F (190 degrees C).
Spread butter onto both sides of each slice of bread, and cut into cubes. Place them in a bag or bowl, and pour the cinnamon sugar over them. Shake or stir to coat. Arrange the bread cubes in a single layer on a baking sheet.
Bake for 10 minutes in the preheated oven, then stir and continue baking for 10 more minutes, until toasted.