Whisk the custard: In a large bowl, whisk eggs, egg whites, Greek yogurt, and milk until smooth. Add protein powder (if using), vanilla, cinnamon, salt, and sweetener. Whisk until no lumps remain.
If thick, add a splash more milk.
Prep the bread: Use slightly stale or toasted bread. Thick slices (about 3/4 inch) hold the custard better and brown nicely.
Heat the pan: Warm a large nonstick skillet or griddle over medium heat. Add half the butter or oil and let it melt and shimmer.
Soak the slices: Dip each slice in the custard for 10–20 seconds per side.
Let excess drip off. The bread should be saturated but not falling apart.
Cook to golden: Lay the soaked slices in the hot pan. Cook 2–3 minutes per side, until deep golden with crisp edges.
Lower heat if they brown too fast.
Finish the batch: Add more butter or oil as needed for the remaining slices. Keep finished toast warm in a low oven (200°F/95°C) while you finish cooking.
Top and serve: Plate with berries, a drizzle of maple syrup, and a dollop of Greek yogurt. For extra protein and healthy fats, add a spoon of peanut or almond butter.