Preheat the oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, cream of tartar, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

In a small bowl, mix together the granulated sugar and ground cinnamon for the topping.

Spread the batter evenly in the prepared baking pan. Sprinkle the cinnamon sugar mixture evenly over the top.

Using a knife or skewer, swirl the cinnamon sugar into the batter.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted in the center comes out clean.

Allow the bars to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

Cut into squares and serve.