Cream together butter, shortening, 1½ cups sugar, eggs, and vanilla. Add in flour, cream of tartar, baking soda, and salt and mix until just combined. Using a large cookie scoop (about ¼ cup), scoop out dough and form into rounded balls.
Mix the 3 tablespoons sugar and cinnamon together. Roll dough balls in mixture to coat. Place cookies on a baking sheet covered in parchment or a silicon baking mat, 6 per sheet. Press down on the balls gently to flatten them slightly.
Bake 7-10 minutes, until just set up on the edges but still a little doughy looking in the middle. Remove from baking sheet and let cool on a cooling rack.
Notes
Source: Adapted from http://allrecipes.com/recipe/mrs-siggs-snickerdoodles/
Recipe by Wright Family Table at https://www.wrightfamilytable.com/snickerdoodles/