Step 1: Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides. Lightly grease the parchment paper.
Step 2: In a small bowl, combine 2 tablespoons of granulated sugar and 2 tablespoons of ground cinnamon for the swirl mixture. Set aside.
Step 3: In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter, remaining 1 1/2 cups granulated sugar, and light brown sugar until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Step 4: Beat in the eggs one at a time, mixing well after each addition. 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: With the mixer on low speed, gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Step 7: Spread about two-thirds of the blondie batter evenly into the prepared baking pan. Sprinkle half of the cinnamon sugar mixture over this layer.
Step 8: Dot the remaining blondie batter over the cinnamon sugar layer, then gently spread it out to cover as much as possible.
Step 9: Sprinkle the remaining cinnamon sugar mixture over the top layer of batter. Use a butter knife or a skewer to gently swirl the cinnamon sugar into the batter, creating a marbled effect.
Step 10: Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with moist crumbs. The center may still look slightly soft.
Step 11: Let the blondies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. Cut into squares and serve.