Step 1: Preheat your oven to 350°F (175°C) for even baking.
Step 2: Grease a 9x13-inch baking pan or line it with parchment paper.
Step 3: In a large bowl, combine melted butter, granulated sugar, light brown sugar, egg, and vanilla extract. Whisk until smooth.
Step 4: Add flour, cream of tartar, and salt. Stir gently until just combined to keep bars soft.
Step 5: In a small bowl, mix together the 1/4 cup granulated sugar and 2 teaspoons of cinnamon for the topping.
Step 6: Pour the batter into the prepared pan and spread evenly.
Step 7: Sprinkle the cinnamon-sugar mixture over the batter.
Step 8: Bake for 20-25 minutes, or until edges are golden and a toothpick comes out clean.
Step 9: Let cool in the pan for 10-15 minutes, then transfer to a wire rack to cool completely before cutting.
