• Preheat the oven to 350 degrees F (or 325 on convection). Prepare cookie sheets with cooking spray or parchment paper.
• Cream together butter, oil, granulated sugar and powdered sugar. Sprinkle the baking soda, cream of tartar, and salt across the top of the sugars. Mix until well-combined and super creamy, 1-2 minutes, scraping down the sides of the bowl as needed.
• Add the sour cream, eggs and vanilla and mix until well-combined, 1-2 minutes, again scraping down the sides of the bowl as needed.
• Add the flour and mix until no dry streaks remain and the mixture is evenly combined; don't overmix.
• Scoop the dough into about balls of about 3 tablespoons. Place several inches apart on the prepared baking sheets. Add about 1/2 cup granulated sugar to a shallow dish or bowl. Lightly spray the bottom of a flat-bottomed glass with cooking spray and dip the bottom of the glass into the sugar. Press each cookie into an even thickness dipping the bottom of the glass into the sugar between each press (no need to spray it again with cooking spray after the first time). Don't press too thin!
• Bake the cookies for 10-12 minutes until just set, not golden brown!
• Let the cookies cool for a few minutes on the baking sheets before removing to a cooling rack to cool completely. Place cooled cookies into the fridge.

Frosting:

• Cream together butter, sour cream, and salt. Add the powdered sugar. Add in the milk or cream to the desired consistency. For best texture, use room temperature frosting on cool cookies from the fridge.