01 - Preheat oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with nonstick spray or butter.
02 - Distribute bread cubes evenly in the prepared baking dish.
03 - In a large bowl, whisk eggs, milk, pumpkin puree, protein powder, maple syrup, vanilla extract, spices, and salt until smooth and combined.
04 - Pour the wet mixture evenly over the bread cubes. Press down gently to ensure thorough soaking.
05 - In a small bowl, mix brown sugar and cinnamon. Sprinkle over the casserole. Add nuts if desired.
06 - Let stand for at least 10 minutes, or cover and refrigerate overnight for enhanced texture.
07 - Bake for 40 minutes, or until the center is set and the top is golden brown.
08 - Cool for 10 minutes before slicing. Serve warm, optionally garnished with maple syrup or Greek yogurt.