Warm a large skillet over medium heat.
In a large bowl, whisk together the egg, egg whites, vanilla protein powder, and cinnamon until smooth.
Place one slice of bread into the mixture and let it soak for 10–15 seconds.
Flip the bread and soak the other side for another 10 seconds.
Repeat with the second slice of bread.
Melt butter in the skillet or lightly coat it with cooking oil.
Transfer the soaked bread slices to the skillet.
Cook the 5 Ingredient Protein French Toast for 4–5 minutes per side, or until golden brown and cooked through.
Serve warm with your favorite toppings such as fresh berries, sliced bananas, Greek yogurt, nut butter, or sugar-free maple syrup.