Prep the bread. Cut day-old bread into 1-inch cubes. If your bread is fresh, spread the cubes on a sheet pan and bake at 300°F (150°C) for 8–10 minutes to dry them slightly. Dry bread soaks up the custard better.

Grease the pan. Lightly coat a 9x13-inch baking dish with cooking spray or a thin layer of butter.

Make the custard base. In a large bowl, whisk eggs, egg whites, milk, Greek yogurt, maple syrup, vanilla, cinnamon, and salt until smooth. You want no streaks of yogurt.

Add the protein powder. Sprinkle in protein powder gradually while whisking to avoid clumps. If it thickens too much, add a splash more milk. The mixture should be pourable, like a thin pancake batter.

Combine with bread. Add bread cubes to the bowl and gently fold until every piece is coated. Let it sit 5–10 minutes so the bread absorbs the custard. If using raisins, nuts, or fruit, fold them in now.

Transfer and level. Pour the mixture into the prepared baking dish and spread it evenly. Lightly press the top to settle the cubes.

Bake. Bake at 350°F (175°C) for 35–45 minutes, until the center is set and the top is golden. A knife inserted in the center should come out mostly clean, with just a little moisture.

Rest before serving. Let it rest 10 minutes. This helps it slice neatly and sets the custard for a creamy, not runny, texture.

Serve. Enjoy warm with a spoonful of Greek yogurt, a drizzle of maple syrup, or a swipe of almond butter. It’s also great plain.