Sift together flour, salt, soda, and cream of tartar, set aside.
In the bowl of an electric mixer, cream together butter and shortening. Add sugars and mix well. Mix in the egg and vanilla until combined. Slowly add flour mixture and mix just until combined.
Drop by heaping tablespoonfuls onto a ungreased baking sheet. Bake at 350°F for 10 to 15 minutes, depending on if you like them light golden brown or a little darker.
Recipe by Not Just Sunday Dinner at https://notjustsundaydinner.com/old-fashioned-sugar-cookies/