1Heat oven to 350°F. Grease bottom only of 8- or 9-inch square pan. In large bowl, beat cake ingredients on low speed 30 seconds. Beat on medium speed 2 minutes. Pour into pan.
2Bake 44 to 49 minutes for 8-inch pan or 38 to 43 minutes or 9-inch pan until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
3In medium bowl, beat powdered sugar, 1/4 cup butter and 1 tablespoon of the orange juice with electric mixer on low speed until mixed. Add remaining orange juice, 1 teaspoon at a time, until creamy and smooth. Beat in 1 teaspoon orange peel. Spread frosting on cooled cake. Garnish with orange peel or mandarin orange segments.