Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with cooking spray or butter. Line it with parchment paper.

Whisk together the melted butter, brown sugar, and granulated sugar in a large mixing bowl until smooth. Add the eggs one at a time, whisking after each, then stir in the vanilla extract.

Sift together the all-purpose flour, cream of tartar, baking soda, and salt in a separate bowl. Gradually mix the dry ingredients into the wet until just combined.

Combine granulated sugar and cinnamon for the cinnamon-sugar swirl, mixing in a bit of melted butter until it resembles damp sand.

Spread half of the blondie batter in the baking dish, sprinkle with the cinnamon-sugar mixture, then layer the remaining batter on top and swirl with a knife.

Bake for 30-35 minutes until edges are golden and a toothpick inserted comes out with moist crumbs. Avoid overbaking.

Cool in the pan for 20 minutes, then using parchment, lift out and slice into squares. Serve warm or at room temperature.