Whisk eggs, protein powder, milk, cinnamon, and salt in a shallow bowl until smooth.

Soak each bread slice in the custard for 15 seconds per side, then let the excess drip off.

Melt 1/2 tablespoon butter in a nonstick skillet over medium heat. Cook two slices at a time for 3 to 4 minutes per side until golden and crisp. Repeat with remaining butter and slices. Serve hot with your favorite toppings.