1. Preheat the oven to 400°F. Line two baking sheets with parchment paper.
2. In a large bowl, cream together the unsalted butter, shortening, and 1½ cups of the sugar. Add the eggs and vanilla extract, and mix until well combined.
3. In a separate medium bowl, whisk together the flour, baking soda, cream of tartar, and salt. Gradually add to the butter mixture. Mix until combined, being careful not to over-mix.
4. In a pie plate, mix together the remaining ¼ cup sugar and the ground cinnamon.
5. Using a cookie scoop, shape the dough into 1½-inch balls. Roll each in the cinnamon-sugar mixture and place on the prepared baking sheets about 2 inches apart. Bake for 7 minutes.
6. Cool on the baking sheets for 2 minutes, then transfer to a wire rack to cool completely before serving.