Step 1: Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides for easy lifting. Grease lightly.
Step 2: In a large bowl, whisk together the melted butter and brown sugar until smooth.
Step 3: Beat in the eggs one at a time, then stir in the vanilla extract until well combined.
Step 4: In a separate medium bowl, whisk together the flour, cream of tartar, baking soda, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Step 6: In a small bowl, combine the granulated sugar and cinnamon for the swirl.
Step 7: Spread about two-thirds of the blondie batter evenly into the prepared baking pan.
Step 8: Sprinkle half of the cinnamon sugar mixture over the batter in the pan.
Step 9: Dollop the remaining blondie batter over the cinnamon sugar. It doesn’t need to be perfectly spread; unevenness will create a better swirl.
Step 10: Sprinkle the remaining cinnamon sugar over the dolloped batter.
Step 11: Take a knife or skewer and gently swirl through the batter to create a marbled effect, ensuring the cinnamon sugar is partially incorporated.
Step 12: Bake for 28-32 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with moist crumbs (not wet batter). Do not overbake.
Step 13: Let the blondies cool completely in the pan on a wire rack before lifting out and cutting into squares. This allows them to set properly and become fudgy.