Heat oven to 350 degrees. Spray a 9x13 pan with cooking spray and set aside. I like to line my pan with parchment paper. It makes it easy to pull out the bars (by using the overhang of the paper) and cut into even slices.
In a stand mixer with the paddle attachment, or a bowl with a hand mixer, combine the butter, oil, granulated sugar, and powdered sugar. Beat until creamy and fluffy looking.
Add egg, water and vanilla and beat until well combined.
Add flour, salt, baking powder and baking soda and blend on low speed just until combined and no flour pockets remain.
Spread cookie dough mixture evenly into the prepared pan. The dough will look crumbly but as you press it into the pan it should come together nicely.
Sprinkle the 1 tablespoon of granulated sugar over the bars before baking. This gives them a sweet sugar crunch that is so good. You can skip it if you want to.
Bake for 12-15 minutes. ** You do not want to overbake these bars or else they will be dry. The bars will still be pale and the edges will look like they've pulled away from the sides of the pan. You don't want them to be golden brown when you pull them out. That means they are overbaked.
Let cool completely before frosting.
TO MAKE FROSTING: Mix butter, sour cream, and powdered sugar together with a blender. Add in the milk or cream until it's smooth and still thick enough to spread over the bars. ** It's best to use heavy cream (for the best texture and flavor) but if you use a lower fat percentage of milk, you will need less of it. I use 3 tablespoons when I make the frosting with the heavy cream.
Frost the cooled cookie bars and decorate with whatever you want. These bars are best served cold. After frosting and decorating, place the pan in the fridge (covered) for 1-3 hours before serving. Leftovers are best stored in the fridge and eaten cold.