Preheat the oven to 375° F. Spray a 13x18-inch pan with nonstick spray.
In a large bowl, use an electric mixer to combine the butter and sugar until fluffy. Add the eggs, one at a time, mixing after each egg. Then mix in the vanilla.
In a separate bowl whisk together the flour, salt and baking soda. Add to the wet mixture and mix just until combined. Spread the sugar cookie dough into the prepared pan. The dough will be just like cookie dough- heavy & sticky- so it's best to spray your hands with a little nonstick spray and use them to help mold the dough into the pan.
Bake for 10 to 15 minutes until light golden brown or until a toothpick comes out clean. Cool completely before frosting.
PREPARE THE FROSTING:
In a large bowl, use an electric mixer to cobmine the butter and shortening until smooth and creamy. Mix in the vanilla and salt. Add the powdered sugar (1 cup at a time) until combined. Then add the milk and mix until it is a smooth, spreading consistency. Add a few drops of color, if desired. Spread over the cooled cookie, then cut into bars.
Notes
TIP: *Refrigerate the bars until firm... it will be easier to get nice, clean cuts.