Preheat your oven to 350°F (175°C). Lightly grease or line a 9x13-inch baking dish with parchment paper for easy removal.
In a large mixing bowl, cream together the butter, margarine, and sugar until light and fluffy. Add the eggs and vanilla and mix until combined.
Stir in the flour, cream of tartar, baking soda, and salt. Mix until just combined—don’t overmix.
Press the dough evenly into the prepared baking dish.
In a small bowl, mix the ¼ cup sugar and cinnamon. Sprinkle evenly over the dough.
Bake for 25–30 minutes, or until the edges are golden and the center is just set. Don’t overbake. Cool completely before cutting into bars.