Preheat oven to 350 degrees Fahrenheit. Line a 9x13-inch baking dish with parchment paper and lightly grease any exposed sides of the pan with cooking spray. (Parchment paper lifts the bars out of the pan for easy frosting, cutting, and serving.)

In a stand mixer with the paddle attachment, cream the butter with the sugars until smooth. Add the oil, water, egg, and vanilla. Mix well.

In a separate bowl, combine the flour, baking soda, cream of tartar and salt. Add all at once to wet ingredients, and mix until everything comes together. It should be slightly crumbly.

Empty cookie dough into the prepared pan and press flat. 

Bake at 350˚F for 16-20 minutes, until the edges turn slightly golden and the cookie dough no longer looks wet in the center. (Do not overbake.) Remove pan from oven and place on a cooling rack.

TO MAKE FROSTING: In a large bowl with an electric hand mixer, beat the butter until smooth and creamy. Add the strawberry preserves and 1 cup of powdered sugar. Blend well. Add the remaining powdered sugar to desired consistency, mixing well. 

Once the bars have cooled, lift up the parchment paper and remove the entire thing from the pan. Spread the frosting evenly over the top. Cut and serve.