1. Preheat the oven to 325 degrees. Mist a 7-by-11-inch baking dish with cooking spray. In a small bowl, combine the melted butter, brown sugar and 1/2 teaspoon of the cinnamon.
2. Drizzle 2 teaspoons of the butter mixture over each slice of bread; spread mixture to the edges. Make three sandwiches; trim away crusts and cut bread into 1/2-inch cubes. Transfer to the baking dish.
3. In a medium bowl, whisk together the eggs, milk, maple syrup, vanilla extract and salt. Pour the mixture over bread and let rest 15 minutes or until most of it has been absorbed. Stir in the diced apple.
4. Combine the granulated sugar with remaining cinnamon; sprinkle over the pudding.
5. Place baking dish in a larger roasting pan and fill pan halfway with very hot water. Bake 30 minutes or until a knife inserted into the pudding comes out clean. Transfer to a wire rack and let cool. Serve.