Preheat the oven to 350 degrees.
Cream butter, shortening, and sugar with an electric hand mixer or stand mixer. Beat in eggs.
In a separate bowl whisk flour, cream of tartar, baking soda, and salt together.
Combine dry ingredients with butter mixture. It will be a soft dough.
Scoop cookies, using a medium cookie scoop, on cookie sheets lined with parchment paper. I usually bake 6 cookies per sheet.
Bake cookies in the oven for 12 minutes.
Allow cookies to cool on baking sheets for 3-5 minutes before transferring them to a cooling rack.