1. Prepare a 9 inch square pan with parchment paper in the bottom and sides, sticking above the edges of the pan, and preheat oven to 350 degrees.
2. Cream butter and sugar together until light and fluffy, about 3-4 minutes.
3. Mix in eggs and McCormick vanilla extract.
4. With the mixer on low speed, add flour, cornstarch, baking soda and baking powder.
5. Press cookie dough into an even layer in the pan.
6. Bake for 19-21 minutes. The center of the cookie will still look a little undercooked, but that’s ok.
7. Remove from oven and allow to cool completely in pan.
8. Use parchment paper at edges of pan to lift bars out of pan.
9. To make the icing, beat the shortening and butter together until smooth.
8. Add the McCormick vanilla extract and mix until smooth.
9. Slowly add the powdered sugar and mix until smooth.
10. Add water or milk as needed.
11. Spread icing onto bars and cut into squares.