Preheat the oven to 350˚F. Lightly grease a 9x13-inch pan. Set aside
In the bowl of a mixer fitted with the paddle attachment, beat together butter and sugars until creamy and fluffy, about 2 minutes. Add eggs and vanilla, beating until combined.
Add flour, cream of tartar, baking soda, and salt. Beat until just combined, making sure not to over mix.
Press the dough into an even layer on the bottom the prepared pan.
In a small bowl, stir together sugar and ground cinnamon for the topping. Sprinkle the cinnamon sugar on top of the dough evenly.
Bake for 22-26 minutes, until just set and the top is a light golden brown. Be sure not to over bake the bars!
Remove and let cool completely in the pan before slicing.
Store in an airtight container at room temperature for up to 3 days.