Preheat the oven to 350 degrees. Line cookie sheets with either silpat or parchment paper.
In a large mixing bowl, or in the bowl of a stand mixer (using the paddle attachment), beat the butter for 2-3 minutes or until light and fluffy. Add in the oil, granulated sugar and powdered sugar and beat until well combined. Add in the eggs, sour cream, and vanilla, mixing well.
In a separate bowl, whisk together flour, baking soda, salt and cream of tartar.
Add the four mixture to the butter and sugar mixture and mix just until flour is completely blended in.
Using a tablespoon or a cookie dough scoop, portion dough into 1 1/2” balls and place at least two inches apart on prepared cookie sheets.
Spray the bottom of a flat-bottom drinking glass with cooking spray and dip into granulated sugar. Use the glass to flatten though dough balls to about ¼” thick.
Bake for 10-12 minutes or until the cookies are set. You do NOT want them to be golden on the edges…this is what keeps them nice and soft.
Cool cookies completely on a wire rack before frosting.
