In large bowl, beat 1/4 cup butter, the milk, orange peel and vanilla with electric mixer on low speed until smooth. Gradually beat in powdered sugar, 1 cup at a time, until smooth and spreadable. If frosting is too thick, beat in more milk, 1 teaspoon at a time. Frost cake. Sprinkle with walnuts. Cut into 3 rows by 3 rows.