Stir together panko, Parmesan, and marjoram on large plate. Whisk egg in small shallow bowl. Place chicken in plastic bag. Using rolling pin, pound to flatten to 1/2-inch thickness. Place chicken on plate; sprinkle with salt and pepper. Brush on all sides with egg. Dredge in panko mixture, turning to coat evenly.
Heat 2 tablespoons oil in heavy large skillet over medium-high heat. Add chicken and cook until deep golden and cooked through, about 4 minutes per side. Transfer chicken to plate. Reduce heat to medium; cool skillet slightly. Add remaining 2 tablespoons oil and vinegar. Stir, scraping up browned bits, until vinaigrette is just hot.
Divide baby greens among 4 plates. Drizzle with vinaigrette and top with chicken. Sprinkle with pepper and serve.
