Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the flour, baking soda, cream of tartar, and salt. Whisk together until well blended. Set aside.
In another bowl, beat together the softened butter, shortening, powdered sugar, and granulated sugar until the mixture is light and fluffy.
Add the eggs and vanilla extract to the creamed mixture. Beat until well combined and smooth.
Gradually add the dry ingredients to the wet mixture, mixing just until combined.
Using a cookie scoop or spoon, drop heaping tablespoons of cookie dough onto the prepared baking sheets.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Once cooled, you can frost them with your favorite icing.
