Ingredients

1 cup unsalted butter (melted and slightly cooled)

1 1/4 cups light brown sugar (packed)

1/2 cup granulated sugar

2 large eggs (room temperature)

2 teaspoons vanilla extract

2 1/4 cups all-purpose flour

1 teaspoon cream of tartar

1/2 teaspoon baking soda

1/2 teaspoon salt

1/3 cup granulated sugar (for the cinnamon swirl)

1 tablespoon ground cinnamon

2 tablespoons unsalted butter (melted for swirl)

2 tablespoons granulated sugar (mixed with 1/2 teaspoon ground cinnamon for topping)

Instructions

Step 1: Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking dish. Line it with parchment paper, leaving an overhang on the sides for easy removal.

Step 2: 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.

Step 3: 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.

Step 4: 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: Spread half of the blondie batter evenly into the prepared baking dish. Sprinkle the cinnamon-sugar mixture over the batter evenly. Dollop the remaining batter on top and gently spread to cover.

Step 6: Using a butter knife, create gentle swirls in the batter to incorporate the cinnamon mixture without fully blending it in.

Step 7: Optionally, sprinkle some of the cinnamon-sugar topping over the surface for added crunch and flavor.

Step 8: Bake 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 9: Allow to cool in the pan for 20 minutes. Lift the blondies out using the parchment paper, transfer to a cutting board, and slice into squares.

Tips & Variations

For extra chewiness, slightly underbake the blondies by checking them a few minutes before the minimum baking time.

Use room temperature eggs to help the batter come together smoothly.

Swap the cream of tartar for an equal amount of baking powder if you don’t have it on hand.

Add chopped nuts or white chocolate chips for extra texture and flavor.

For a stronger cinnamon flavor, increase the cinnamon in the swirl mixture by half a teaspoon.

FAQs

Can I use salted butter instead of unsalted?

Yes, but reduce the added salt in the recipe to avoid over-salting. Typically, omit the 1/2 teaspoon of salt if using salted butter.

Why is cream of tartar used in this recipe?

Cream of tartar helps create the classic snickerdoodle flavor and contributes to a tender, chewy texture by reacting with baking soda as a leavening agent.