Panko-crusted Chicken with Mustard-maple Pan Sauce

Using meat mallet or rolling pin, pound chicken in resealable plastic bag to 1/3-to 1/2-inch thickness. Whisk egg, parsley, and 2 teaspoons Dijon mustard in large bowl. Place chicken in egg mixture; turn to coat. Sprinkle chicken with salt and pepper. Dip each chicken piece in panko; turn to coat. Heat oil in large nonstick skillet over medium-high heat. Add chicken; cook until brown and cooked through, about 4 minutes per side. Meanwhile, whisk broth, syrup, coarse-grained mustard, and remaining 2 tablespoons Dijon mustard in glass measuring cup. Transfer chicken to plates. Add broth mixture to skillet; boil until reduced to 3/4 cup, stirring occasionally, about 4 minutes. Add butter; whisk until melted. Spoon sauce alongside chicken.