In a large bowl, whisk together the eggs, egg whites, milk, protein powder, cinnamon, and vanilla.
You can transfer the batter to a shallow container or keep it in the bowl. Using tongs, carefully dip each slice of bread into the batter, coating both sides.
Allow the excess liquid to drip back into the bowl, then place the soaked slices of bread onto a plate.
Spray a nonstick griddle or skillet with cooking spray or olive oil over medium-high heat.
Place one slice of bread (or two if two fit) onto the pan and cook for about 3 minutes, then flip over and cook the other side for about 3 minutes, just until both sides are golden brown.