Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a large mixing bowl, cream the softened butter and 1 1/2 cups sugar using a hand mixer or stand mixer. Beat for 4-5 minutes until light and fluffy. Add eggs and vanilla, then mix for another 1-2 minutes.

In a separate bowl, whisk together flour, cream of tartar, baking soda, and salt.

Gradually add the dry mixture to the wet mixture. Mix on low speed until just combined. Do not overmix.

In a small bowl, mix 1/4 cup sugar with 1 1/2 tablespoons cinnamon for the coating.

Optional: Chill dough for 20-30 minutes for easier handling.

Scoop dough into 1-2 tablespoon-sized balls. Roll in cinnamon-sugar mixture twice for full coating. Optionally flatten slightly.

Place coated balls on baking sheet, spaced 2 inches apart. Bake 9-11 minutes until edges are set and centers soft.

Cool on baking sheet for a few minutes before transferring to wire rack to cool completely.