In medium bowl, beat powdered sugar, 1/4 cup softened butter, 2 tablespoons of the milk and 1/4 teaspoon maple extract with spoon until smooth and fluffy. If frosting is too stiff to spread, add additional milk, 1 teaspoon at a time. Frost top of bar. Sprinkle 1/4 cup walnuts on top. Cut into 4 rows by 4 rows.