Make the custard. In a large bowl, whisk eggs, milk, vanilla extract, cinnamon, and salt.
Sprinkle in the protein powder gradually, whisking until smooth with no lumps. If needed, use a blender for a silky mixture.
Check sweetness. Taste the custard. If your protein powder isn’t sweet, add a little maple syrup or preferred sweetener and whisk again.
Preheat the pan. Heat a large nonstick skillet or griddle over medium heat.
Add a small pat of butter or a light swipe of oil so the surface is coated.
Soak the bread. Dip each slice into the custard for 10–15 seconds per side. Let excess drip off. Thick or stale bread may need a few extra seconds; avoid soaking so long that it falls apart.
Cook to golden. Place the soaked slices on the hot pan.
Cook 2–3 minutes per side, until golden brown and slightly crisp on the edges. Adjust heat if it’s browning too fast.
Serve warm. Stack on plates and add your favorite toppings. For more protein, top with Greek yogurt or a dollop of cottage cheese.
For a crowd. Keep finished slices warm on a baking sheet in a 200°F (95°C) oven while you cook the rest.