Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a large mixing bowl, cream the softened butter and shortening until light and fluffy.
Step 3: Gradually add the granulated sugar, then add the eggs one at a time, mixing well after each addition, and stir in the vanilla extract.
Step 4: In another bowl, whisk together the flour, cream of tartar, baking soda, and salt.
Step 5: Slowly add the dry mixture to the wet mixture, stirring until just combined.
Step 6: In a small bowl, mix the granulated sugar and ground cinnamon for the topping.
Step 7: Grease or line a 9x13 inch baking pan, then spread the batter evenly in the pan.
Step 8: Sprinkle the cinnamon sugar mixture over the top of the batter.
Step 9: Bake for 20-25 minutes or until the edges are golden and a toothpick comes out clean.
Step 10: Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before cutting into squares.
