Preheat oven to 350°F. Spray an 8×8-inch or 9×9-inch baking dish with non-stick cooking spray. Set aside.
In a large bowl, blend the brown sugar and melted butter on high speed until smooth (about 5 minutes). Add the eggs and vanilla and mix on medium speed until thoroughly combined and creamy.
In another bowl, whisk together the flour, baking powder and salt. Slowly add the flour mixture to the mixer while beating on low speed. When almost completely blended, turn off the mixer and stir in the chocolate chips by hand.
Press the mixture into the prepared pan and bake for 24-18 minutes or until the edges are browned and the center is set. Allow to cool in the pan for 15 minutes before turning out onto a wire rack to cool completely. Once cool, flip the bars right-side up on a cutting board and cut into 8 rectangles.
While the cookie bars are baking, make the frosting. In the bowl of a mixer, combine the shortening, vanilla and salt. Cream on med-high speed for 2 minutes. Scrape the sides of the bowl. Add the powdered sugar, one cup at a time, and blend into the shortening mixture on low-med speed. Add the milk, one tablespoon at a time, until you have reached your desired consistency.
Decorate bars with frosting and candy eyes.