Adjust oven rack to middle position and heat oven to 350 degrees. Remove crusts from bread and cut into 1/2-inch cubes (you should have about 1 1/4 cups). Toss cubes with melted butter. Combine sugar with cinnamon, and toss bread cubes with mixture. Spread cubes in even layer on parchment-lined baking sheet; bake until crisp, 8 to 10 minutes.