In large bowl, beat 1 cup butter with electric mixer on medium speed about 2 minutes or until creamy. Beat in 2 cups powdered sugar. Add eggs, one at a time, beating well after each addition. Add flour mixture to butter mixture alternately with milk, beating well on low speed after each addition. Beat on low speed 1 minute. Stir in lemon juice, lemon peel and zucchini. Spoon into pan.