Lightly coat a 9 x 13-inch baking dish with cooking spray. If the bread is very soft and fresh, let it sit uncovered while you make the custard.

Arrange six slices across the bottom of the dish. Trim or slightly overlap the bread as needed.

Add the eggs, vanilla protein shake or milk, protein powder, sugar alternative, cinnamon, vanilla, and salt to a blender. Blend until smooth. With the blender running on low, slowly stream in the melted and slightly cooled butter.

Pour about half the custard over the first six slices, lifting the corners so some liquid reaches underneath. Add the remaining six slices, pour the rest of the custard over the top, and press gently so both layers make contact with the liquid.

Cover and refrigerate for at least 2 hours, preferably overnight. Before baking, spoon custard from the sides over any dry areas on top.

Preheat the oven to 350°F. Remove the casserole from the refrigerator while the oven heats, then uncover and bake for 45 to 55 minutes. It is ready when the top is golden, the center is set, and the middle reaches 160°F. If the top browns before the center sets, loosely cover the pan with foil for the remaining bake time.

Let the casserole rest for 10 to 15 minutes before cutting it into six large portions. Serve warm.