Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line with parchment paper with an overhang.
Step 2: Brown the butter. In a light-colored saucepan, melt the 1 cup of butter over medium heat. Continue to cook, swirling occasionally, until the butter foams, then subsides, and golden-brown bits appear at the bottom. It will smell nutty. Immediately remove from heat and pour into a heatproof bowl. Let cool for 10 minutes.
Step 3: In a large bowl, whisk together the slightly cooled brown butter, 1 cup granulated sugar, and light brown sugar until well combined.
Step 4: Beat in the eggs one at a time, mixing well after each addition, until the mixture is light and fluffy. Stir in the vanilla extract.
Step 5: In a separate medium bowl, whisk together the all-purpose flour, cream of tartar, baking soda, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix.
Step 7: Prepare the cinnamon swirl mixture: In a small bowl, combine the 2 tbsp granulated sugar and 1 1/2 tsp ground cinnamon.
Step 8: Spread about two-thirds of the blondie batter evenly into your prepared baking pan.
Step 9: Sprinkle about half of the cinnamon sugar mixture over the batter. Drop spoonfuls of the remaining blondie batter over the cinnamon layer. Sprinkle the rest of the cinnamon sugar over the top.
Step 10: Use a knife or a skewer to gently swirl the cinnamon mixture into the batter. Don't overmix; aim for distinct ribbons.
Step 11: Bake for 30-35 minutes, or until the edges are set and lightly golden, and a toothpick inserted into the center comes out with moist crumbs. Do not overbake to keep them chewy.
Step 12: Allow the Brown Butter Cinnamon Swirl Snickerdoodle Blondies to cool completely in the pan on a wire rack before slicing and serving. This ensures perfect texture.