Heat oven to 350°F. Grease bottom and sides of 15x10x1-inch pan with shortening; lightly flour (or spray with baking spray with flour).
In large bowl, beat bar ingredients with electric mixer on low speed, scraping bowl occasionally, until moistened. Beat on medium speed 2 minutes, scraping bowl occasionally. Spread batter evenly in pan.
Bake 23 to 27 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
In 2-quart saucepan, heat butter over medium heat, stirring constantly, until light golden brown. Remove from heat. Stir in powdered sugar, vanilla and enough milk until smooth and spreadable. Immediately spread frosting over cooled bars. Refrigerate about 15 minutes or until set. For bars, cut into 8 rows by 6 rows. If desired, garnish each bar with orange peel strip.