Step 1: Preheat the Oven

Preheat your oven to 350°F (175°C). Line a 9×13-inch baking dish with parchment paper, leaving an overhang on the sides for easy removal. Lightly grease the parchment with nonstick spray or butter.

Step 2: Mix the Wet Ingredients

In a large mixing bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth. Add the eggs and vanilla extract, whisking until fully combined. The mixture should be thick and glossy.

Step 3: Combine the Dry Ingredients

In a separate bowl, whisk together the flour, cream of tartar, baking soda, and salt.

Step 4: Make the Blondie Batter

Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Be careful not to overmix, as this can make the blondies dense. The batter will be thick.

Step 5: Prepare the Cinnamon-Sugar Topping

In a small bowl, mix the granulated sugar and cinnamon for the topping.

Step 6: Assemble the Blondies

Spread the batter evenly into the prepared baking dish using a spatula. Sprinkle the cinnamon-sugar mixture evenly over the top, ensuring it reaches the edges.

Step 7: Bake

Bake in the preheated oven for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. The edges should be lightly golden, and the center should be set but soft.

Step 8: Cool and Slice

Allow the blondies to cool in the pan for at least 20 minutes. Use the parchment overhang to lift the blondies out of the pan. Slice into squares and serve.