Step 1: Preheat the Oven

Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish and line it with parchment paper, leaving an overhang on the sides for easy removal.

Step 2: Mix the Wet Ingredients

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

Step 3: Combine the Dry Ingredients

In a separate bowl, sift together the flour, cream of tartar, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix, as this can lead to dense blondies.

Step 4: Prepare the Cinnamon Swirl

In a small bowl, mix the granulated sugar and cinnamon for the swirl. Stir in the melted butter until the mixture resembles wet sand.

Step 5: Layer the Batter and Swirl

Spread half of the blondie batter evenly into the prepared baking dish using a spatula. Sprinkle the cinnamon-sugar mixture over the batter, spreading it evenly across the surface. Dollop the remaining batter over the cinnamon-sugar layer and gently spread it to cover. Use a butter knife to create swirls by dragging it through the batter in a zigzag pattern.

Step 6: Add the Optional Topping

If desired, sprinkle the cinnamon-sugar topping evenly over the surface for an extra spiced crunch.

Step 7: Bake

Bake the blondies in the preheated oven for 30–35 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.

Step 8: Cool and Slice

Allow the blondies to cool in the pan for 20 minutes. Use the parchment paper overhang to lift the blondies out of the pan, then transfer them to a cutting board. Slice into squares and serve.