Preheat oven to 350℉ and grease a 9x13 pan with cooking spray; set aside.
In the bowl of a stand mixer, cream the softened butter and sugar.
1 cup butter, 1 1/2 cups sugar
Add the eggs and vanilla. Scrape down the sides of the bowl with a rubber spatula and mix to combine.
2 eggs, 2 teaspoons vanilla extract
Add the flour, cream of tartar, baking soda and salt and mix until combined.
3 cups flour, 1 1/2 teaspoons cream of tartar, 1 teaspoon baking soda, 1/2 teaspoon salt
Press about half of the dough into the bottom of the prepared pan.
In a separate small bowl, mix together the sugar and cinnamon.
1/4 cup sugar, 1 teaspoon cinnamon
Sprinkle 2 tablespoons of the cinnamon sugar mixture over the pan of dough.
Spoon the remaining dough over the cinnamon sugar and use greased hands (or a piece of greased wax paper) to spread the dough.
Sprinkle the remaining cinnamon sugar mixture on top.
Bake for 20-22 minutes or until very lightly golden brown around the edges; do NOT overbake.
Let cool then cut into bars and enjoy!